refactor(proto): destructure lambda expression serde hooks

[buraksenn]

Which issue does this PR close?

• Closes Destructure proto hooks for lambda physical expressions #24617.

Rationale for this change

Proto hooks that access fields individually can silently omit newly added state. Exhaustive destructuring makes such omissions compile errors.

What changes are included in this PR?

• Exhaustively destructure LambdaExpr and LambdaVariable in their encoders.
• Exhaustively destructure their protobuf nodes in the decoders.
• Document constructor-derived state and add focused proto hook tests.

The behavior and wire format are unchanged.

Are these changes tested?

Covered by the added lambda proto hook tests:

cargo test -p datafusion-physical-expr --features proto lambda::proto_tests
cargo test -p datafusion-physical-expr --features proto lambda_variable::proto_tests

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.24561% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.53%. Comparing base (61bf6b9) to head (eea7b16).

Files with missing lines	Patch %	Lines
...n/physical-expr/src/expressions/lambda_variable.rs	97.14%	1 Missing and 1 partial ⚠️
datafusion/physical-expr/src/expressions/lambda.rs	99.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #24775    +/-   ##
========================================
  Coverage   81.52%   81.53%            
========================================
  Files        1123     1123            
  Lines      405970   406133   +163     
  Branches   405970   406133   +163     
========================================
+ Hits       330978   331139   +161     
- Misses      55627    55630     +3     
+ Partials    19365    19364     -1     

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