fix: cancel disconnected upstream streams

[orangeboyChen]

Summary

• cancel and unlock upstream readers when streaming clients disconnect
• release readers after normal stream completion
• cover Responses passthrough, Responses adapter, and Anthropic adapter cancellation

Validation

• bun run test -- tests/server/units.test.ts tests/server/anthropic.test.ts
• bun run test:coverage
• bun run test:ci
• bun run lint
• bun run format:check
• bun run typecheck
• bun run build

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.06%. Comparing base (b624efa) to head (57b80c6).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   92.03%   92.06%   +0.03%     
==========================================
  Files          20       20              
  Lines        8131     8251     +120     
  Branches     1624     1644      +20     
==========================================
+ Hits         7483     7596     +113     
- Misses        635      642       +7     
  Partials       13       13              

Flag	Coverage Δ
unittests	92.06% <100.00%> (+0.03%)	⬆️

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