fix: validate PROTO/ENUM wire kind in literal cast plugins

[apstndb]

Summary

• Call requireStringWire in FormatProtoAsCast and FormatEnumAsCast after NULL handling so malformed structpb kinds return ErrUnknownType instead of emitting invalid SQL.
• Add regression tests for non-string wire kinds (bool/number) on PROTO and ENUM via LiteralFormatConfig.

Fixes #156.

Test plan

• make check
• New TestFormatProtoEnumCastRejectsNonStringWire covers PROTO/ENUM bool and number wire kinds

Made with Cursor

[gemini-code-assist]

Code Review

This pull request adds validation checks in common.go to ensure that Proto and Enum values are represented as string wire types before formatting them as casts, using requireStringWire in FormatProtoAsCast and FormatEnumAsCast. It also adds unit tests in common_test.go to verify that non-string wire types are correctly rejected with ErrUnknownType. There are no review comments, and I have no feedback to provide.

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[apstndb]

LGTM. Calling requireStringWire after the IsNull guard in FormatProtoAsCast/FormatEnumAsCast is the right placement, and returning ErrUnknownType for non-string PROTO/ENUM wire matches how validateScalarWire already classifies those codes (both route to requireStringWire), so the slow/scalar paths stay consistent. Previously a bool/number wire would silently produce CAST(b"" AS ...) / CAST( AS ...) invalid SQL, so this is a clear correctness fix. The table test covers bool and number kinds for both PROTO and ENUM via LiteralFormatConfig. No issues found.