feat: add User-Agent header with version, OS, and arch

[deanq]

Summary

Add comprehensive User-Agent header to all HTTP transmissions in the flash library for better API analytics, debugging, and deployment environment visibility.

Changes

• New module: user_agent.py - Centralized User-Agent string generation
• Updated centralized HTTP utilities: Modified get_authenticated_httpx_client() and get_authenticated_requests_session() to include User-Agent
• Updated aiohttp clients: Added User-Agent to both RunPodApi and RunPodClient session creation
• Updated direct requests calls: Added User-Agent to file upload/download operations in app.py
• Comprehensive test coverage: Added 11 new tests (8 for user_agent module, 4 for HTTP utilities)

User-Agent Format

Runpod Flash/<version> (Python <python_version>; <OS> <OS_version>; <arch>)

Example: Runpod Flash/1.1.1 (Python 3.11.12; Darwin 25.2.0; arm64)

Components

• Flash version: Package version from metadata
• Python version: Runtime Python version
• OS: Operating system name (Darwin, Linux, Windows)
• OS version: Operating system version/release
• Architecture: CPU architecture (arm64, x86_64, amd64, etc.)

Test Results

• All 23 User-Agent tests pass (8 user_agent + 15 http utility tests)
• Coverage: 69.17% (above 65% threshold)
• Linting and formatting checks pass

Impact

• Non-breaking: User-Agent is informational and doesn't affect API functionality
• Backward compatible: All existing functionality preserved
• Performance: Negligible overhead (string concatenation)
• Value: Provides deployment environment insights for debugging and analytics

Benefits

1. API Analytics: Track flash adoption, version distribution, and platform usage
2. Debugging: Identify client environment in server logs for troubleshooting
3. Compliance: Follows HTTP best practices for client identification
4. Platform Insights: Understand deployment patterns across OS/architecture combinations