fix(committor): propagate tx assembly errors in prepare_for_strategy

[crazywriter1]

Summary

TransactionPreparatorImpl::prepare_for_strategy used .expect() when assembling the final transaction message. A prior feasibility check runs against dummy lookup tables, so assembly can still fail with the real prepared data — causing the committor worker to panic.

This change replaces .expect() with ? so assembly failures are propagated as errors instead of panicking.

Fixes #1018

Test plan

• cargo check -p magicblock-committor-service
• cargo test -p magicblock-committor-service

Summary by CodeRabbit

• Bug Fixes
  • Transaction preparation now reports message assembly errors gracefully instead of terminating unexpectedly.
  • Improved error propagation allows callers to handle failed transaction preparation safely.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 7f56741d-bbce-4e89-96af-828b89c2919b

📥 Commits

Reviewing files that changed from the base of the PR and between e5119fa and 154b61b.

📒 Files selected for processing (1)

• magicblock-committor-service/src/transaction_preparator/mod.rs

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📝 Walkthrough

Walkthrough

TransactionPreparatorImpl::prepare_for_strategy now propagates TransactionUtils::assemble_tasks_tx errors through PreparatorResult using ?. Successful assembly continues to extract the VersionedMessage.

Assessment against linked issues:

Objective	Addressed	Explanation
Propagate transaction assembly errors instead of panicking in prepare_for_strategy [#1018]	✅

Suggested reviewers: taco-paco, gabrielepicco

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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