pipeline_manager: refactor register_process into focused helper methods (#642)

[tiagonix]

Here's the PR description text:

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📌 Summary

Refactor register_process into focused helper methods to reduce cyclomatic complexity and improve maintainability of the pipeline task identification logic.

🔍 Related Issues/Tickets

Part of ongoing work to improve pipeline manager code quality.
Closes #642

✨ Changes Introduced

Decomposes the monolithic register_process method into three private helpers:

• update_existing_candidates: Handles filtering and deduplication when adding PIDs to existing candidate matches
• identify_new_candidates: Isolates candidate discovery logic including specialized rule matching
• find_best_match: Extracts scoring, threshold checking, and tie-break selection

This reduces complexity in a critical code path while preserving all existing behavior and semantics.

✨ Infrastructure Impact

None. This is a behavior-preserving refactor with no changes to public APIs or external interfaces.

✅ Checklist

• Code follows the project's style guidelines
• I have performed a self-review of my own code
• I have tested the changes and they work as expected
• Documentation has been updated if needed
• Tests have been added or updated

🛠️ How to Test

Run existing unit tests:

cargo test --package tracer pipeline_manager

All tests should pass unchanged, demonstrating behavior preservation.

📌 Additional Notes

This refactor intentionally preserves existing behavior including:

• Current tie-break semantics (higher total_rules wins on equal scores)
• Use of unwrap() for task lookups
• Exact same scoring and filtering logic

Follow-up work (deferred to separate PR):

• Add unit tests for find_best_match tie-break behavior
• Evaluate tie-break semantics and document intent
• Consider replacing unwrap() with graceful error handling