server: unify handling of unary and streaming RPCs (#9309)

This change unifies the processing for unary and streaming RPCs on the
gRPC-Go server. By treating all RPCs fundamentally as streams
internally, this change simplifies the codebase and eliminates duplicate
logic.

Summary of changes:
* Merged `processUnaryRPC` and `processStreamingRPC` into a single
`processRPC` method by wrapping unary handlers as stream handlers.
* Removed the methods map from `serviceInfo`, storing all descriptors in
the streams map.
* Ensured interceptor segregation is maintained so that unary
interceptors only run for unary RPCs and stream interceptors only run
for streaming RPCs.
* Introduced thread-safe, exactly-once status writing in `serverStream`
using an atomic boolean to prevent duplicate status writes.
* Added tests to verify interceptor segregation and concurrent status
writing behavior.

RELEASE NOTES: none