refactor: replace manual mocks with mockall in producer

[raushan728]

Migrated manual mock implementations in producer/src/mocks.rs to mockall to address internal TODOs and standardize testing infrastructure.

[fuel-cla-bot]

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PR Summary

Standardizes producer test infrastructure by replacing hand-rolled mocks with mockall and small factory helpers.

• Introduces mockall-based mocks for Relayer, TxPool, and Executor in producer/src/mocks.rs, plus helpers: create_mock_relayer, create_mock_txpool, create_mock_executor, and create_failing_mock_executor (replaces FailingMockExecutor)
• Updates tests to use the new helpers and mock types, including TestContext and TestContextBuilder construction paths
• Keeps MockExecutorWithCapture and MockDb test utilities; no production logic changes beyond a minor formatting fix in block_producer.rs

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[xgreenx]

Can you please apply nightly formatting?

[raushan728]

Applied nightly formatting as requested. All imports are now in vertical layout format.

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

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[MitchTurner]

Hey @raushan728 !

Thanks for the contribution! It's a little bit of an internal debate whether or not we should use mockall in our code. I'm of mind that we should manually write our mocks as mockall encourages users to pay too much attention to implementation details, rather than just testing external apis.

It's not up to me alone, but that's my position.

[raushan728]

Thanks @MitchTurner. I'll wait for the team's final decision on this before making further changes.