fix: recover streaming responses after network interruption

[Praneeth2711]

Summary

Fixes Issue #243 by making streaming responses resilient to temporary network interruptions.

Changes

• Added support for resume_offset in chat requests.
• Implemented background stream buffering to allow reconnecting clients to continue receiving the same response.
• Added automatic frontend retry logic (up to 3 attempts).
• Preserved partial assistant responses instead of replacing them with connection errors.
• Added TTL cleanup for completed and abandoned stream buffers.
• Added protection against duplicate user messages during reconnects.
• Added SQLite fallback for completed streams.

Testing

• Added regression tests covering:

  • Normal stream completion
  • Client disconnect during generation
  • Reconnect during active generation
  • Reconnect after generation completion
  • TTL cleanup behavior

Verification

• Backend tests passed.
• Frontend tests passed.
• Frontend production build completed successfully.

Closes #243.

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[imDarshanGK]

@Praneeth2711 resolve conflicts

[Praneeth2711]

Resolved merge conflicts with upstream/main, fixed all Ruff lint issues, and pushed the updates.

Verification:

• Backend tests passing
• Frontend build passing
• Frontend CI passing
• Security scan passing
• Streaming recovery functionality preserved
• Benchmark reporting preserved

The branch is ready for review and merge.