fix(proxy): capture streaming token usage for passthrough providers

[SebConejo]

Summary

Fixes two bugs causing streaming responses to log 0 tokens in the dashboard and return 0 tokens to downstream clients (breaking context management in agents like OpenClaw):

• Passthrough buffer flush: pipeStream() in stream-writer.ts never flushed the passthrough buffer after the stream ended. If the final SSE chunk containing usage didn't end with \n\n, it stayed unparsed and tokens were logged as 0. Added a flush block mirroring the existing transform-path flush.

• stream_options injection: The proxy didn't inject stream_options.include_usage: true into outgoing requests for OpenAI/OpenRouter. If the calling agent didn't send this field, the provider wouldn't include usage in the stream — so neither Manifest's DB nor the downstream client got token counts. Now injected automatically for streaming requests to passthrough providers.

Transform paths (Google/Anthropic/ChatGPT adapters) were unaffected — they already synthesize usage chunks.

Closes #1477, closes #1502

Test Coverage

• 4 new tests for passthrough buffer flush (usage capture, non-JSON, [DONE], whitespace-only)
• 5 new tests for stream_options injection (OpenAI, OpenRouter, non-streaming, non-passthrough, merge with existing options)
• All 3553 backend tests pass

Pre-Landing Review

No issues found.

Test plan

• All backend tests pass (3553 tests, 181 suites)
• New tests cover all new code paths
• Adversarial review: no actionable findings

🤖 Generated with Claude Code

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Summary by cubic

Fixes zero-token logs for streaming responses by capturing usage from passthrough providers. Adds a passthrough buffer flush and injects stream_options.include_usage for openai/openrouter streaming requests to ensure correct token counts in the dashboard and for downstream clients.

• Bug Fixes
  • Flushes leftover passthrough SSE buffer in pipeStream() to parse final usage chunks without trailing \n\n.
  • Injects stream_options.include_usage: true for streaming requests to openai and openrouter while preserving existing stream_options.
  • Adds tests for buffer flush and injection logic; transform adapters already synthesize usage and are unaffected.

^{Written for commit 4442ecc. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.94%. Comparing base (34ab548) to head (4442ecc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1567   +/-   ##
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  Coverage   97.94%   97.94%           
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  Files         118      118           
  Lines        8616     8616           
  Branches     3242     3242           
=======================================
  Hits         8439     8439           
  Misses        175      175           
  Partials        2        2           

Flag	Coverage Δ
frontend	97.93% <ø> (ø)
shared	100.00% <ø> (ø)

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[cubic-dev-ai]

No issues found across 4 files