[codex] Complete protovalidate coverage

[jrandolf]

Summary

• generate AsCelValue impls across files when CEL expressions reference nested messages
• add oneof validation for scalars, enums, messages, wrappers, supported WKTs, and CEL rules
• add CEL conversions for supported WKT values and stricter compile-time CEL type checks

Validation

• cargo +stable fmt --all --check
• cargo +stable check -p protoc-gen-protovalidate-buffa -p protovalidate-buffa
• cargo +stable clippy -p protoc-gen-protovalidate-buffa --all-targets
• cargo +stable clippy -p protovalidate-buffa --all-targets
• cargo +stable test -p protovalidate-buffa
• cargo +stable build -p protovalidate-buffa-conformance
• protovalidate-conformance target/debug/protovalidate-buffa-conformance
• protovalidate-conformance --strict_error target/debug/protovalidate-buffa-conformance