chore: scope mutable-key lint exceptions for window expressions

[VVKot]

Which issue does this PR close?

Validation prerequisite for the semi-join optimization draft.

Rationale for this change

Clippy reports mutable-key-type on three existing window-expression sets because Expr contains interior-mutable memory reservations. Those reservations do not participate in equality or hashing.

What changes are included in this PR?

Allow mutable-key-type at the three affected declarations and document why their keys remain stable.

What is the testing strategy for this PR?

All-target, all-feature Clippy passes. This changes no executable behavior; existing window pushdown tests cover the affected code.

Are there any user-facing changes?

None.

AI-assisted contribution authored with Codex.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.80%. Comparing base (75ddead) to head (e7628f7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26207      +/-   ##
==========================================
- Coverage   82.80%   82.80%   -0.01%     
==========================================
  Files        1149     1149              
  Lines      452205   452205              
  Branches   452205   452205              
==========================================
- Hits       374458   374451       -7     
- Misses      54950    54955       +5     
- Partials    22797    22799       +2     

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