chore: migration trace - rich error model client API

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Motivation

This PR tracks the migration of upstream akka/akka-grpc commits 1c08d131 and 40fb2f2c which together add first-class client-side support for the gRPC Rich Error Model.

Previously, when a gRPC call failed with a StatusRuntimeException containing rich error details (as defined by Google's API Design Guide error model), there was no first-class API to access the structured com.google.rpc.Status from the client side. Users had to manually inspect trailers using grpc-java internals.

Note: This feature was independently ported to apache/pekko-grpc as PR #645 (merged as commit 82300037). This PR exists solely for migration audit trail purposes.

Modification

• Added scaladsl.MetadataStatus trait with status, code, message, details, getParsedDetails[K] methods
• Added javadsl.MetadataStatus trait with Java-style getter methods
• Added RichGrpcMetadataImpl (internal) that extracts com.google.rpc.Status from StatusRuntimeException trailers via StatusProto.fromStatusAndTrailers
• Changed GrpcServiceException.apply(StatusRuntimeException) to wrap metadata in RichGrpcMetadataImpl
• Added GrpcServiceException.create() Java API accepting java.util.List
• Added .recoverWith(richError) / .recoverWithRetries(1, richErrorStream) to all RequestBuilderImpl invocation paths
• New RichErrorModelNativeSpec.scala and RichErrorModelNativeTest.java integration tests
• RichErrorModelSpec refactored from abstract to concrete

Result

Clients can pattern-match GrpcServiceException.metadata as MetadataStatus and call getParsedDetails[LocalizedMessage] (or similar) to access structured error details without touching grpc-java internals.

References

• Upstream commit 1 (which is now Apache licensed): akka/akka-grpc@1c08d131
• Upstream commit 2 (which is now Apache licensed): akka/akka-grpc@40fb2f2c
• Upstream PR: Rich error model client api akka/akka-grpc#1665 and feat: Better name for rich status metadata, better details lookup API akka/akka-grpc#1740
• Already merged in pekko-grpc as: Rich error model client API #645
• Our local migration commit: 7fe45e6b

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Investigation: Why this PR has no file changes

After investigation, this PR contains zero file changes because the Rich Error Model client API feature was already independently merged into apache/pekko-grpc as PR #645 (commit 82300037).

Our migration commit 7fe45e6b on branch port-2.3.0 implements the same feature ported from akka/akka-grpc commits 1c08d131 and 40fb2f2c. Since PR #645 was already merged before we could submit our migration PR, there are no new changes to contribute.

Closing this PR as the feature is already available in main. The migration is tracked in our migration-checklist.md for audit purposes.