Fix nested-lambda variable collision

[koenbeuk]

Resolve variable name collisions in nested lambdas by incorporating a unique prefix for each lambda's parameters. This change prevents compile errors related to duplicate variable declarations in generated interceptors. Additional tests ensure that the generated code compiles correctly under these conditions.

[Copilot]

Pull request overview

Fixes a codegen correctness issue in ExpressionTreeEmitter where nested lambda parameter locals could collide across sibling lambdas emitted into the same interceptor body, causing CS0128 duplicate-variable compile errors. The change aligns nested-lambda parameter variable naming with the existing per-lambda _varPrefix scheme used elsewhere in the interceptor generator.

Changes:

• Prefix nested-lambda ParameterExpression local variable names with the per-lambda _varPrefix to ensure uniqueness across sibling lambda emissions.
• Add a regression test that compiles the generated interceptor for a Join with sibling key selectors that each contain a nested lambda with the same parameter name.
• Update verified snapshot outputs to reflect the new prefixed parameter variable names.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/ExpressiveSharp.Generator.Tests/PolyfillInterceptorGenerator/JoinTests.cs	Adds a regression compilation test covering sibling key-selector lambdas with nested-lambda parameter name collisions.
tests/ExpressiveSharp.Generator.Tests/PolyfillInterceptorGenerator/ExecuteUpdateTests.ExecuteUpdateAsync_GeneratesInterceptor.verified.txt	Updates snapshot to reflect prefixed nested-lambda parameter variable names.
tests/ExpressiveSharp.Generator.Tests/PolyfillInterceptorGenerator/ExecuteUpdateTests.ExecuteUpdate_SetProperty_WithSwitchExpression.verified.txt	Updates snapshot to reflect prefixed nested-lambda parameter variable names.
tests/ExpressiveSharp.Generator.Tests/PolyfillInterceptorGenerator/ExecuteUpdateTests.ExecuteUpdate_SetProperty_WithNullConditional.verified.txt	Updates snapshot to reflect prefixed nested-lambda parameter variable names.
tests/ExpressiveSharp.Generator.Tests/PolyfillInterceptorGenerator/ExecuteUpdateTests.ExecuteUpdate_SetProperty_ConstantValue.verified.txt	Updates snapshot to reflect prefixed nested-lambda parameter variable names.
src/ExpressiveSharp.Generator/Emitter/ExpressionTreeEmitter.cs	Ensures nested lambda parameter locals include _varPrefix, preventing cross-emitter collisions in shared interceptor bodies.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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