starknet_transaction_prover: add prover input validation unit tests

[avi-starkware]

Summary

• Add 7 unit tests for validate_transaction_input: pending block rejection, non-invoke transaction types, proof/proof_facts field checks, fee field enforcement, and fee validation toggle
• Add test-only with_fee_validation builder method on VirtualSnosProver

Test plan

• SEED=0 cargo test -p starknet_transaction_prover -- virtual_snos_prover_test — 7 pass, 2 ignored (integration)

🤖 Generated with Claude Code

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Note

Low Risk
Low risk because changes are test-only plus a test-time toggle for fee-field validation; no production proving logic is modified beyond how tests construct the prover.

Overview
Adds a focused set of unit tests around VirtualSnosProver::prove_transaction input validation (rejects Pending block IDs, non-InvokeV3 tx types, non-empty proof/proof_facts, and non-zero fee-related fields when validation is enabled).

Introduces lightweight mock runners (UnreachableRunner, FailingRunner) to assert whether validation short-circuits before OS execution, and updates the ignored Sepolia integration tests to call .disable_fee_validation() so they exercise the full pipeline without fee-field constraints.

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[reviewable-StarkWare]

This change is 

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[avi-starkware]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• starknet_transaction_prover: add deployment smoke test script #13159
• starknet_transaction_prover: add manual and deployment smoke testing guides #13158
• starknet_transaction_prover: add TLS integration tests #13138
• starknet_transaction_prover: add CORS and config validation tests #13136
• starknet_transaction_prover: add exhaustive error enum and error mapping tests #13135
• starknet_transaction_prover: add committer utils unit tests #13134
• starknet_transaction_prover: add prover input validation unit tests #13133 👈 (View in Graphite)
• starknet_transaction_prover: sanitize error messages returned to clients #13132
• starknet_transaction_prover: add ConfigMap docs and full config reference #13167
• starknet_transaction_prover: add external developer docs #13129
• starknet_transaction_prover: validate request inputs before proving #13125
• main-v0.14.2

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