Fix conversion gen cross package conversions - #143367

What type of PR is this?

/kind bug
/sig api-machinery

What this PR does / why we need it:

When a type references a struct from another API group (for example authorization/v1alpha1.AuthorizationConditionsRequest -> admission/v1.AdmissionRequest, or AuthorizationConditionsResponse -> authorization/v1.ConditionsAwareDecision), conversion-gen only looked for hand-written Convert_* functions. The functions it generates itself into the other package live in zz_generated.conversion.go, which is not parsed, so it emitted compileErrorOnMissingConversion(). The workaround was a hand-written wrapper that just calls autoConvert_*. Those wrappers count as manual conversions, so every type that contains the wrapped type lost its memory-identical (unsafe cast) conversion.

This PR:

1. conversion-gen: before generating, records the public Convert_X_To_Y functions that will be generated into each input package (types with a peer, convertible, and without a manual conversion). When a conversion is needed between types from another package, conversion-gen calls that function instead of emitting compileErrorOnMissingConversion(). These functions are kept separate from the manual conversions, so they don't block memory-identical conversions and aren't registered with AddConversionFunc.
2. Cleanup: removes the wrappers in pkg/apis/admission/v1/conversion.go and pkg/apis/authorization/v1/conversion.go and regenerates. ConditionsAwareDecision, NamedConditionsAwareDecision, SubjectAccessReviewStatus and AuthorizationConditionsResponse now convert with an unsafe cast, and SubjectAccessReview, SelfSubjectAccessReview and LocalSubjectAccessReview are removed from the TestMemoryIdenticalConversion exemptions (part of Conditional Authz [3/n]: Make the webhook authorizer support conditions #137633, as requested in the issue). AuthorizationConditionsReview stays exempt because admission.AdmissionRequest has runtime.Object fields.

Which issue(s) this PR is related to:

Fixes #142345

Special notes for your reviewer:

• AI tools were used to help explore the generator code and draft the change and the unit test. I reviewed every line, re-ran the generators and tests listed below, and can explain each change.
• The two commits are split into the generator change and the regenerated code, so the first one can be reviewed on its own. With the wrappers still present, the first commit produces no generated diff.
• Only packages generated in the same conversion-gen run are considered, which is how hack/update-codegen.sh runs it (all tagged packages in one invocation). Packages outside the run behave as before.
• If the peer package can't generate the public function (a field needs manual conversion), the caller now references an undefined Convert_* function instead of compileErrorOnMissingConversion(). Both fail to compile, and adding the manual function fixes both.
• Tested with hack/verify-codegen.sh, hack/verify-golangci-lint.sh, hack/verify-gofmt.sh, hack/update-codegen.sh conversions and hack/update-codegen.sh subprojects (no diff in the subprojects or code-generator examples), go test ./staging/src/k8s.io/code-generator/cmd/conversion-gen/... (new TestCrossPackageGeneratedConversions), go test ./pkg/api/testing/... (including TestMemoryIdenticalConversion and the round-trip tests), and go test ./pkg/apis/authorization/... ./pkg/apis/admission/... ./pkg/registry/authorization/....

Does this PR introduce a user-facing change?

Fixed conversion-gen to call the conversion functions it generates for types in other packages of the same run, instead of emitting `compileErrorOnMissingConversion()`. Hand-written wrappers that only call the autogenerated `autoConvert_*` function are no longer needed; removing them lets types that contain the wrapped type use memory-identical conversions.

AI usage disclosure:

YES. This PR was written in part with the assistance of generative AI (exploring the generator code, drafting the change and the unit test). The solution was reviewed and tested by me (Yash), and I am responsible for the submitted changes.