fix: respect Retry-After header on HTTP 429 responses (#938)

[ayash911]

Fixes #938

The Kaggle CLI was ignoring the Retry-After header on HTTP 429 responses. This PR:

• Adds _is_rate_limited() and _get_retry_after_delay() to detect 429s and parse the Retry-After header (supports both integer seconds and HTTP-date formats per RFC 9110)
• Updates with_retry() and download_file() to use the server-specified delay instead of blind exponential backoff
• Adds verbose rate-limit logging via self.logger (enabled with VERBOSE=true)
• Includes 15 new unit tests — all passing, 0 warnings

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[ayash911]

@googlebot I signed it.