feat: Add construct-meta-transaction command (#562)

There was no way to explicitly construct and sign a meta-transaction
(delegate action) — the only path was implicit via
`meta_transaction_relayer_url` in network config. This adds `transaction
construct-meta-transaction` as an explicit command, and consolidates the
delegate-action decision into a single context flag so signers don't
check two places.

## Key changes

- **New `construct-meta-transaction` command** — same CLI structure as
`construct-transaction` (sender, receiver, action chain); reuses
`add_action_1`/`2`/`3`/`skip_action` entirely without duplication;
outputs `ConstructTransactionContext` with `sign_as_delegate_action:
true`

- **`sign_as_delegate_action: bool` flag** propagated through the
context chain:
- `ConstructTransactionContext` → `ActionContext` (via `skip_action`) →
`NetworkForTransactionArgsContext` → `TransactionContext`
- Set in `NetworkForTransactionArgsContext` as
`context.sign_as_delegate_action ||
network_config.meta_transaction_relayer_url.is_some()`, so the relayer
URL config is translated into the flag — signers now check one place
only

- **All signers updated** (`sign_with_keychain`,
`sign_with_legacy_keychain`, `sign_with_private_key`,
`sign_with_seed_phrase`, `sign_with_access_key_file`,
`sign_with_ledger`) to check `previous_context.sign_as_delegate_action`
instead of `network_config.meta_transaction_relayer_url.is_some()`

- **`submit_dao_proposal` and `sign_with_mpc`** now derive
`sign_as_delegate_action` from
`network_config.meta_transaction_relayer_url.is_some()`, preserving
existing behavior for those flows when a relayer URL is configured

- **`send` step** now returns a user-facing error instead of panicking
when a `SignedDelegateAction` is produced but no
`meta_transaction_relayer_url` is configured (e.g. when using
`construct-meta-transaction` without a relayer), with a message guiding
the user to configure the relayer URL or use `transaction
send-meta-transaction`

Example usage:
```sh
near transaction construct-meta-transaction alice.near bob.near \
  add-action transfer 1NEAR \
  skip \
  network-config mainnet \
  sign-with-keychain display
```

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[Task] Add `construct-meta-transaction`</issue_title>
> <issue_description>### Background
> 
> Meta-transactions on near are basically just a transaction with a
single action
[`DelegateAction`](https://github.com/near/nearcore/blob/master/core/primitives/src/action/delegate.rs).
> 
> The `DelegateAction` is basically a `SignedTransaction` with
`max_block_height` which controls the expiration of the signed payload.
> 
> near-cli-rs already has support for creating and sending
Meta-transactions if the network config has
`meta_transaction_relayer_url`, so currently signer implementations
(e.g.
[1](https://github.com/near/near-cli-rs/blob/b067abdefa0306a13395e512f5453dda4b3579d4/src/transaction_signature_options/sign_with_legacy_keychain/mod.rs#L195))
make the decision based on this config setting, but there is no way to
explicitly create and sign the Delegate Action.
> 
> ### User Story
> 
> As a NEAR CLI user, who uses dApps that support meta-transactions and
use NEAR Connect with NEAR CLI plugin, I need CLI to support
construction of meta-transactions just like `construct-transaction`.
> 
> ### Acceptance Criteria
> 
> I would like to have a new subcommand under `transaction`, so next to
`construct-transaction` we could have `construct-meta-transaction`, the
`construct-transaction` implementations should be reused without
copy-pasting them, and the only change that is expected is to the
signers implementation that would use the context flag to make the
decision whether they should construct & sign a transaction or delegate
action. As for the `meta_transaction_relayer_url` config option, it
should be translated into the context flag setting as well, so signers
don't need to check two places for the decision.
> 
> ### Resources & Additional Notes
> 
> _No response_</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #561

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: frol <304265+frol@users.noreply.github.com>
Co-authored-by: Vlad Frolov <frolvlad@gmail.com>

Author: 3 people

src/commands/account/add_key/autogenerate_new_keypair/print_keypair_to_terminal/mod.rs

@@ -78,6 +78,7 @@ impl From<PrintKeypairToTerminalContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 move |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/autogenerate_new_keypair/save_keypair_to_keychain/mod.rs

@@ -75,6 +75,7 @@ impl From<SaveKeypairToKeychainContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/autogenerate_new_keypair/save_keypair_to_legacy_keychain/mod.rs

@@ -97,6 +97,7 @@ impl From<SaveKeypairToLegacyKeychainContext> for crate::commands::ActionContext
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/use_ledger/mod.rs

@@ -90,6 +90,7 @@ impl From<AddLedgerKeyActionContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/use_manually_provided_seed_phrase/mod.rs

@@ -74,6 +74,7 @@ impl From<AddAccessWithSeedPhraseActionContext> for crate::commands::ActionConte
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/use_mpc/mod.rs

@@ -239,6 +239,7 @@ impl From<MpcDeriveKeyToAddContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/add_key/use_public_key/mod.rs

@@ -67,6 +67,7 @@ impl From<AddAccessKeyActionContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/create_account/fund_myself_create_account/sign_as/mod.rs

@@ -154,6 +154,7 @@ impl From<SignerAccountIdContext> for crate::commands::ActionContext {
             ),
             on_before_sending_transaction_callback: item.on_before_sending_transaction_callback,
             on_after_sending_transaction_callback,
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/delete_account/mod.rs

@@ -106,6 +106,7 @@ impl From<BeneficiaryAccountContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }

src/commands/account/delete_key/public_keys_to_delete.rs

@@ -74,6 +74,7 @@ impl From<PublicKeyListContext> for crate::commands::ActionContext {
             on_after_sending_transaction_callback: std::sync::Arc::new(
                 |_outcome_view, _network_config| Ok(()),
             ),
+            sign_as_delegate_action: false,
         }
     }
 }