fix: retry synchronous failures from async delegates

[sylvesterkaczmarek]

Summary

Route synchronous exceptions thrown by HttpClient.executeAsync() through RetryingHttpClient's existing async retry machinery instead of letting them escape before retry handling is attached.

Fixes #888.

Problem

RetryingHttpClient.executeAsync() currently calls its delegate directly:

val responseFuture = httpClient.executeAsync(requestWithRetryCount, requestOptions)

A custom HttpClient can throw synchronously from executeAsync() rather than returning an exceptionally completed future. In that case, the exception escapes before the existing handleAsync retry path runs.

That means synchronous IOException, OpenAIIoException, and OpenAIRetryableException failures bypass maxRetries entirely, even though the same failures are retryable when delivered asynchronously.

Fix

Wrap the delegate call and convert a synchronous throwable into an exceptionally completed CompletableFuture<HttpResponse>.

The resulting future then flows through the existing retry handler unchanged, so:

• retryable synchronous failures use the normal backoff and retry-count logic;
• non-retryable synchronous failures are returned through the async API as failed futures;
• no duplicate retry policy is introduced.

Regression coverage

Added focused tests for:

1. a synchronous OpenAIRetryableException followed by a successful response, verifying two attempts and retry-count headers 0 then 1;
2. a synchronous non-retryable exception, verifying executeAsync() itself returns normally with an exceptionally completed future and no retry/backoff occurs.

Validation

• branch is based on current upstream main at cf942a40074291290634321ad9fe21e514030b4c;
• branch is 0 commits behind upstream;
• production diff is 8 additions / 1 deletion in RetryingHttpClient.kt;
• retry classification, backoff, response handling, and synchronous execute() behavior are unchanged.

Full repository validation is left to GitHub Actions.

Risk

Low. Existing delegates that already return futures are unaffected. The behavior changes only when an async delegate throws before returning a future.