Enhance DeepSeek streaming, uploads, and testing support

[3mora2]

Summary

This PR modernizes the authenticated DeepSeek provider to support the current web chat protocol, including reliable SSE streaming, proof-of-work challenges, file uploads, stream recovery, and comprehensive tests.

What changed

Streaming and response recovery

• Added a dedicated SSE parser that preserves DeepSeek event types and supports snapshots, patches, batches, and close events.
• Added support for continuing INCOMPLETE responses and resuming interrupted streams.
• Tracks fragment types and indexes, including indexed SET, APPEND, /type, /content, and -1 patches.
• Emits THINK, search, and tool fragments as reasoning while keeping RESPONSE and TEMPLATE_RESPONSE as visible output.
• Prevents duplicate content when a stream is resumed or replaced by a full-message snapshot.
• Attempts one bounded recovery when DeepSeek closes with FINISHED but provides no visible response.
• Raises a clear ResponseError if the recovered message still contains no response instead of silently returning an empty result.
• Never promotes hidden reasoning content to the final response.

File uploads and proof of work

• Added endpoint-specific DeepSeek PoW challenge generation and solving.
• Runs the CPU-intensive PoW solver outside the async event loop.
• Added multipart file uploads with MIME type and filename handling.
• Supports multiple uploaded files while preserving their order.
• Waits for server-side file parsing and handles terminal parsing failures explicitly.
• Updated detect_file_type to consistently return the detected extension and MIME type.

Provider structure and protocol handling

• Moved PoW and streaming logic into dedicated deepseek helper modules.
• Added normalized chat and upload headers.
• Added explicit completion payload construction and business-response envelope validation.
• Updated chat session creation, deletion, upload, completion, continue, and resume handling to match the observed DeepSeek web contract.
• Improved diagnostic logging without exposing response content or authentication data.

Testing and diagnostics

• Added unit coverage for SSE parsing, fragment classification, status handling, resume and continue flows, empty responses, PoW, uploads, file parsing, and error responses.
• Added a reusable JSONL chunk logger for inspecting provider metadata and reasoning chunks during development.

Why

DeepSeek streams responses as incremental fragment patches and can occasionally close a message as FINISHED after emitting reasoning without a final response. Previously, this could result in a successful request with an empty visible answer.

The updated implementation recovers the complete message when possible and returns an explicit error when DeepSeek genuinely provides no final response.

Validation

• python -m unittest etc.unittest.test_deepseek_stream etc.unittest.test_deepseek_upload
  • 40 tests, OK
• python -m etc.unittest
  • 235 tests run
  • 9 skipped
  • Suite OK

Final verification was performed locally without issuing an additional live DeepSeek request.