Add cross-language round-trip contract tests — catch serialization mismatches before production

[haasonsaas]

Problem

The existing contract test (contract_test.go) validates that Go's protojson uses UseProtoNames: true (snake_case field names). This catches one class of issue, but misses cross-language mismatches:

• evalops/chat#948: @bufbuild/protobuf v1 generates different TypeScript types than v2
• evalops/fermata#9239: Python protobuf version produces incompatible serialization

These bugs surface at runtime in production when a Go service sends a proto-JSON payload that a TypeScript or Python consumer can't deserialize — or deserializes with silently wrong values.

Proposed solution

1. Cross-language round-trip tests

For each message type with a contract fixture:

Go: serialize fixture → JSON
TypeScript: deserialize JSON → re-serialize → JSON₂
Python: deserialize JSON → re-serialize → JSON₃
Assert: JSON == JSON₂ == JSON₃

This catches:

• Field name casing mismatches (camelCase vs snake_case)
• Enum serialization differences (string vs int)
• Timestamp format differences
• oneof handling differences
• Default value presence/absence

2. Fixture generation from proto

Generate random valid messages using proto reflection:

func FuzzRoundTrip(f *testing.F) {
    f.Fuzz(func(t *testing.T, data []byte) {
        msg := &governancev1.EvaluateActionRequest{}
        if err := proto.Unmarshal(data, msg); err != nil {
            return // invalid proto, skip
        }
        json, _ := protojson.MarshalOptions{UseProtoNames: true}.Marshal(msg)
        // ... deserialize in TS and Python, compare
    })
}

3. CI integration

Add a test job that:

1. Installs Go, Node.js, and Python
2. Runs the round-trip tests against gen/go, gen/ts, gen/python
3. Fails the proto PR if any language produces different serialization

Relationship to existing work

• Extends the existing contract_test.go pattern
• Complements the contractfixtures/ directory (fixtures become the test corpus)
• Catches the class of bugs from chat#948 and fermata#9239 at PR time instead of production