Planner: Add comprehensive debug logging for query planner NoRewrite cases

[systay]

Description

This PR adds detailed debug logging throughout the Vitess query planner to explain why specific optimizations are not performed when DebugOperatorTree = true. Previously, developers only saw that rewrites failed without understanding the specific reasons.

The implementation adds a debugNoRewrite() helper function and comprehensive logging to 50+ NoRewrite cases across key planner functions including horizon push/expand, limit optimization, ordering, filtering, distinct operations, and join merging. Each debug message provides specific context about blocking conditions such as phase restrictions, dependency mismatches, routing incompatibilities, or structural limitations.

This greatly improves the debugging experience for query plan optimization, making it much easier to understand why certain optimizations fail and how to address performance issues in distributed queries.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[arthurschreiber]

❤️