[sdk-platform-java] gax: Consider removing @BetaApi and/or adding @InternalExtensionOnly to retry surface

Decide whether to add @InternalExtensionOnly to the following retry elements, and whether to remove @BetaApi

https://github.com/googleapis/sdk-platform-java/blob/decd7f60017d45288e6c12493fa21e06f9c5f77c/gax-java/gax/src/main/java/com/google/api/gax/retrying/ScheduledRetryingExecutor.java#L98

https://github.com/googleapis/sdk-platform-java/blob/decd7f60017d45288e6c12493fa21e06f9c5f77c/gax-java/gax/src/main/java/com/google/api/gax/retrying/RetrySettings.java#L309

https://github.com/googleapis/sdk-platform-java/blob/decd7f60017d45288e6c12493fa21e06f9c5f77c/gax-java/gax/src/main/java/com/google/api/gax/retrying/RetryingContext.java#L44

https://github.com/googleapis/sdk-platform-java/blame/decd7f60017d45288e6c12493fa21e06f9c5f77c/gax-java/gax/src/main/java/com/google/api/gax/retrying/RetryAlgorithm.java#L258

https://github.com/googleapis/sdk-platform-java/blame/decd7f60017d45288e6c12493fa21e06f9c5f77c/gax-java/gax/src/main/java/com/google/api/gax/retrying/RetryAlgorithm.java#L264

Start by reviewing the referenced lines in ScheduledRetryingExecutor.java, RetrySettings.java, RetryingContext.java, and RetryAlgorithm.java, then compare how @BetaApi and @InternalExtensionOnly are used across the retry surface. Determine the intended API stability policy for these elements; the issue is done when that decision is recorded and the corresponding annotations are consistently updated.

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