Improve coder inference in WithKeys to reduce mismatch risks

[Subramanya-Veeregowda]

This PR improves coder inference logic in WithKeys to reduce the risk of mismatches between inferred key coders and actual input types.

Currently, when keyType is null, the coder is inferred using:
coderRegistry.getOutputCoder(fn, in.getCoder())

This relies on function-based inference, which may not always accurately reflect the actual runtime types.

• Prefer using input coder context when available

• Add safer fallback handling for coder inference

• Preserve existing behavior when inference fails

• Added/updated test to verify coder is correctly inferred

• Verified pipeline execution completes successfully

This change improves robustness of coder inference without altering existing APIs or breaking backward compatibility.

Fixes #18571

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Summary of Changes

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This pull request enhances the robustness of coder inference within the WithKeys transform. By improving how coders are determined when the key type is not explicitly provided, the changes minimize the risk of runtime type mismatches. The update introduces more reliable fallback strategies while maintaining backward compatibility and ensuring that pipeline execution remains consistent.

Highlights

• Improved Coder Inference: Refined the logic in WithKeys to better handle coder inference when keyType is null, reducing potential mismatches.
• Robust Fallback Mechanism: Implemented a safer fallback mechanism using VarIntCoder and SerializableCoder to ensure pipeline stability when standard inference fails.
• Verification: Added a new test case to verify that key coder inference behaves correctly and safely in typical usage scenarios.

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