Fix Retry-After header overflow error (closes #1600)

[n1ghter]

Problem

When Jira Server or Confluence Server returns HTTP 429 (Rate Limiting) with a Retry-After header containing a large value, the library throws an OverflowError: timestamp too large to convert to C _PyTime_t error.

This fixes issue #1600 and related issues #1499 and #1593.

Solution

• Added _parse_retry_after_header method that safely parses Retry-After headers (supports both delta-seconds and HTTP-date formats)
• Clamps header values to max_backoff_seconds to prevent overflow
• Updated _retry_handler to use the safe parser instead of direct int() conversion
• Added comprehensive tests for edge cases

Changes

• atlassian/rest_client.py: Added safe Retry-After parsing with overflow protection
• tests/test_rest_client.py: Added tests for large values, HTTP-date parsing, and invalid headers

Testing

• All existing tests pass
• New tests verify overflow protection works correctly
• Docker image built and tested with mcp-atlassian integration