feat: enable Phase 3-5 for FULL JOIN to collect unmatched LEFT rows

[poyrazK]

Summary

• Track outer_join_left_key in addition to outer_join_right_key
• Enable Phase 3-5 for FULL JOIN (not RIGHT JOIN - RIGHT works via local executor)
• Add LEFT-side Phase 3: UnmatchedLeftRowsReport RPC to each data node
• Add LEFT-side Phase 4: FetchUnmatchedLeftRows RPC to collect unmatched rows
• Aggregate unmatched LEFT rows into final results

Test plan

• All relevant tests pass locally
• CI passes

Implementation Notes

This implements Option C symmetric handling for FULL JOIN:

• Unmatched RIGHT rows: collected via existing Phase 3-4
• Unmatched LEFT rows: collected via new LEFT-side Phase 3-4

Part of Option C implementation for FULL JOIN support.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 26e81439-0fac-4150-8d36-60fded731965

📥 Commits

Reviewing files that changed from the base of the PR and between b3e07d1 and 52740b0.

📒 Files selected for processing (2)

• include/network/rpc_message.hpp
• src/distributed/distributed_executor.cpp

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/full-join-option-c-orchestration

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[poyrazK]

It's okay to merge