…figurable backoff

Enhances RetryMiddleware in Guzzle HTTP client to support rate-limit awareness and customizable retry logic:

Recognizes and honors the Retry-After header (numeric and HTTP-date formats) for accurate wait times.

Adds configurable maxRetries and baseDelay (in ms) for exponential backoff control.

Retains existing support for connection errors and server (5xx/429) retries.

Why It Matters
This upgrade delivers smarter and more resilient HTTP clients:

Adapts to server-imposed delays (Retry-After), reducing unnecessary retries.

Offers flexible defaults — maxRetries = 3, baseDelay = 100 ms — while allowing overrides.

Reduces boilerplate: developers no longer need to build custom retry logic from scratch.

Key Changes

Modified src/RetryMiddleware.php:

Introduced maxRetries and baseDelay parameters.

Enhanced __invoke() flow to parse Retry-After header and implement delay logic.

Ensured delay continues via exponential backoff when Retry-After isn’t present.

Updated tests/RetryMiddlewareTest.php:

Added new test testRespectsRetryAfterHeader to validate Retry-After handling.

Ensured retries occur as expected when the header exists.

Test Coverage
Includes existing and new tests verifying:

Default retry behavior on connection failures and server errors.

Correct delay and retry count in exponential backoff scenarios.

Respect for Retry-After, ensuring the middleware waits properly before retrying.

Compatibility
Fully backward-compatible. No breaking changes—just enhancements layered onto existing behavior.