fix: clamp negative Retry-After delays before sleeping

[abhinavkr26104]

Description

RetryingHttpClient does not clamp negative Retry-After values to zero before passing them to the configured Sleeper. A server-provided past HTTP date, negative numeric delay, or negative millisecond delay can therefore make an otherwise retryable request fail instead of retrying immediately.

Reproduction

Configure a retrying client with the default sleeper and return a retryable response containing a past HTTP-date:

// The date is already in the past when the response is handled.
Retry-After: Wed, 21 Oct 2015 07:28:00 GMT

Or return either of these headers:

Retry-After: -1
Retry-After-Ms: -1

Then make a request that receives a retryable 5xx/429 response.

Code reference

• openai-java-core/src/main/kotlin/com/openai/core/http/RetryingHttpClient.kt:197-218 parses numeric and HTTP-date values and returns Duration.ofNanos(retryAfterNanos.toLong()) without clamping.
• RetryingHttpClient.kt:72 passes the duration to synchronous Sleeper.sleep().
• RetryingHttpClient.kt:121 passes the duration to asynchronous Sleeper.sleepAsync().
• openai-java-core/src/main/kotlin/com/openai/core/DefaultSleeper.kt:13-22 calls Thread.sleep(duration.toMillis()) or schedules a TimerTask directly.

Expected behavior

Negative retry delays should be treated as zero, so the retry proceeds immediately. This is the usual interpretation for an HTTP-date that has already elapsed and avoids making clock skew or malformed negative delay values fatal.

Actual behavior

The synchronous default sleeper throws IllegalArgumentException from Thread.sleep for a negative duration. The asynchronous default sleeper can throw from Timer.schedule, causing the retry future to fail instead of issuing the next attempt.

Why it matters

Retry-After HTTP dates are especially vulnerable to client/server clock skew and network transit time. A valid retryable response can therefore become a non-retryable SDK failure, affecting both synchronous and asynchronous callers.

Suggested fix

Clamp parsed retry durations to Duration.ZERO before returning them, and add sync/async regression coverage for past dates and negative numeric headers.