[SantoshSarma]

🔗 Related Issues

Fixes #18129

💥 What does this PR do?

Fix generated Java BiDi types rejecting valid special-number wire strings, emitting raw enum objects from NumberValue.toMap(), accepting explicit null for non-nullable fields, and losing explicit-null presence for optional nullable fields.

Adds regressions for all four SpecialNumber wire values, ordinary numbers, invalid union values, SharedReference.handle:null rejection, and Info.parent null-versus-absent presence.

🔧 Implementation Notes

• Coerce a string-valued enum arm before inline-union constructor validation, and serialize enum union arms through toWireValue().
• Generate a fromJson decoder for each record to enforce required-key presence and field nullability before Optional conversion loses that information.
• Pass map.containsKey(wire) through nullable-field SetOverride parameters, including receiver-only records.
• Preserve extension capture and unknown-field warnings in custom decoders; leave the shared ConstructorCoercer unchanged.

🤖 AI assistance

• No substantial AI assistance used
• AI assisted (complete below)
  • Tool(s): OpenAI Codex
  • What was generated: generator implementation changes, regression tests, and this PR description; Codex also ran build and test commands.
  • I reviewed all AI output and can explain the change

💡 Additional Considerations

Validation:

• Reproduced four failing regression methods on the original generator.
• 16 small BiDi test targets passed: 69 test cases, no failures or skips. After rebasing onto current trunk, the three changed test targets reran and passed; the other 13 targets reused passing cached results.
• All 480 targets under //java/src/... built successfully.
• Selenium's ./scripts/format.sh --pre-push passed (Buildifier, copyright, and Java format); git diff --check passed.
• Validation used the installed macOS SDK 26.5 via a local repository override, matching the requested SDK version, and --pin_browsers=false to avoid stalled browser downloads. No dependency files changed; browser integration tests were not run.

A comprehensive generator unit-test suite for #18128 remains separate from this bug fix.

🔄 Types of changes

• Bug fix

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ SantoshSarma
❌ santhosh-sarma-0007
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[diemol]

@SantoshSarma The CLA needs to be signed, please.