fix: handle ValueError from null bytes and replace sys.exit with exceptions (fixes #319, #317)

[KooshaPari]

Fixes #319, fixes #317

Changes

• Added input validation to reject null bytes in range_spec
• Added range format validation (empty parts, empty spec)
• Replaced sys.exit(1) in extract_issue_numbers() with raised exceptions (ValueError, RuntimeError)
• Error handling moved to __main__ block, making the function safe for import by MCP servers
• Added empty result handling (returns empty string instead of empty list)

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