Experiment with AST paths

[systay]

Description

Proof of Concept: Path-Based Semantic State Keys

This PR is an experimental proof of concept exploring the use of paths (rather than references) as keys for the semantic state.
1. Currently, using references as map keys forces a copy of the semantic state during cloning.
2. By switching to paths, the AST can be cloned without duplicating semantic state.
3. If the AST is rewritten, updated paths would be added to the semantic state—no different from the existing approach.

Note: This is purely experimental and not intended for merging.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 12.03704% with 95 lines in your changes missing coverage. Please review.

Project coverage is 67.65%. Comparing base (d1aa2f4) to head (c7330f6).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/sqlparser/ast_path.go	12.03%	95 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17607      +/-   ##
==========================================
- Coverage   67.68%   67.65%   -0.04%     
==========================================
  Files        1586     1587       +1     
  Lines      255170   255278     +108     
==========================================
- Hits       172722   172713       -9     
- Misses      82448    82565     +117     

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