core: prevent RetriableStream registry leak on cancel race

[014-code]

When ClientCall.cancel() races ClientCall.start() on a retry-enabled channel, RetriableStream.cancel() can commit the stream before start() calls prestart().

The commit callback then removes nothing because the stream has not been registered yet. prestart() subsequently registers the already-committed stream, and the one-shot commit prevents any later removal. This permanently retains the stream in UncommittedRetriableStreamsRegistry and can delay channel shutdown.

This change makes RetriableStream.start() check whether the stream was committed after prestart(). If so, it runs postCommit() again after registration, allowing the registry to remove the stream.

Added regression coverage for:

• retry streams;
• hedging streams;
• streams without retry or hedging policy.

Related issue: #13034

Testing:

• git diff --check

• Attempted:

  ./gradlew :grpc-core:test --tests io.grpc.internal.RetriableStreamTest -PskipAndroid=true -PskipCodegen=true
  

[linux-foundation-easycla]

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• ✅ login: 014-code / name: Donson (8ace0ad)