fix: return protobuf map key conversion errors

[fallintoplace]

Summary

• Make protobuf map key conversion return an error instead of panicking for unsupported key kinds.
• Thread conversion errors through map iteration into record construction with contextual message including field name and map type.
• Add regression tests for invalid map key conversion and contextual append error path.

Validation

• Changes committed in branch: fix-protobuf-map-key-error-clean
• Target: apache/arrow-go:main
• No tests run in this step.