Fix ValueError when parsing malformed rate limit header

[IFAKA]

Problem

When the API returns a 429 status with a malformed X-RateLimit-Reset header (e.g., empty string, float, or non-numeric value), the SDK crashes with an unhandled ValueError:

# client.py:156
retry_after=int(retry_after) if retry_after else None
# If retry_after is "invalid" or "12.5", int() raises ValueError

Fix

Wrap the integer conversion in a try/except block to gracefully handle malformed headers:

if retry_after_header:
    try:
        retry_after = int(retry_after_header)
    except (ValueError, TypeError):
        pass  # Keep retry_after as None

Test

Test 1: Malformed header 'invalid'
  ✓ RateLimitError raised correctly
  ✓ retry_after=None (None expected)

Test 2: Valid header '60'
  ✓ RateLimitError raised correctly
  ✓ retry_after=60 (60 expected)

Test 3: Float header '12.5'
  ✓ RateLimitError raised correctly
  ✓ retry_after=None (None expected)

Test 4: Empty header ''
  ✓ RateLimitError raised correctly
  ✓ retry_after=None (None expected)

Test 5: Missing header
  ✓ RateLimitError raised correctly
  ✓ retry_after=None (None expected)

==================================================
All tests passed!