txnbuild: Add txnbuild helper for sending a payment to a smart contract

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Close https://github.com/stellar/go/issues/5626

Update txnbuild to support constructing SAC payments to contracts with default fees so that simulateTransaction calls to rpc are not necessary.

Also fixes a bug in txnbuild where the max fee in the transaction envelope did not include the additional soroban resource fee.

Why

see https://github.com/stellar/go/issues/5626

Known limitations

[N/A]

[Copilot]

Pull Request Overview

This PR adds txnbuild helpers for sending a payment to a smart contract and for restoring an asset balance via a new restore footprint operation. It also updates fee handling and adjusts integration tests to use the new helper signatures while removing legacy fee parameters.

• Introduces NewAssetBalanceRestoration and NewPaymentToContract functions with configurable Soroban fee values.
• Updates integration tests to use the new Preflight functions without separate minFee values.
• Refactors code across txnbuild and integration tests to align with these helper changes.

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated no comments.

File	Description
txnbuild/restore_footprint.go	Adds a helper for constructing asset balance restoration operations.
txnbuild/invoke_host_function.go	Adds a helper for constructing payment-to-contract operations with updated fee handling.
Integration tests (various files)	Adjust test calls to use the new helper function signatures by removing extra fee parameters.
Horizon integration tests	Update fee-bump and Preflight function calls to support the new txnbuild APIs.

Comments suppressed due to low confidence (2)

services/horizon/internal/integration/integration.go:816

• Double-check that all callers of PreflightHostFunctions have been updated to handle the new signature returning a single value, as the removal of minFee could cause runtime issues if any outdated expectation remains.

return function

services/horizon/internal/integration/invokehostfunction_test.go:130

• [nitpick] Consider defining a named constant for 'HostFunctionTypeHostFunctionTypeUploadContractWasm' to improve code clarity and reduce potential typos in repeated usage.

assert.Equal(t, invokeHostFunctionOpJson.Function, "HostFunctionTypeHostFunctionTypeUploadContractWasm")

[tamirms]

@sreuland can you take another look at the PR?

[sreuland]

lgtm, nice work!