fix(claude): clear Anthropic passthrough deadline after headers

[MustangRider]

Summary

• replace the native Anthropic passthrough's non-clearable
  AbortSignal.timeout() with the existing clearableDeadline() helper
• clear the deadline as soon as upstream response headers arrive
• keep the response body linked to the client request signal
• add a regression test whose SSE body outlives connectTimeoutMs

Fixes #135

Problem

connectTimeoutMs is a response-header arrival budget, but the native
Anthropic passthrough kept its timeout signal attached to the entire response
body.

As a result, a valid SSE response could be terminated after the deadline even
when its headers arrived immediately.

Test coverage

The new endpoint regression test uses a local mock Anthropic upstream:

• response headers and message_start arrive immediately
• connectTimeoutMs is 200 ms
• message_stop arrives after 600 ms

Before this change, reading the response body ends at the timeout and the
final event is missing. After this change, the complete SSE body arrives.

Verification

• focused regression test: pass
• tests/claude-messages-endpoint.test.ts: 12 passed
• typecheck: pass
• privacy scan: pass
• runtime proxy verification:
  • body completed after 681 ms with a 200 ms header deadline
  • delayed response headers still returned 504 after approximately 200 ms

The full Windows test run also reported three unrelated pre-existing
environment/path failures in codex-v2-gate.test.ts and doctor.test.ts;
the Claude endpoint test file passed in full.

🤖 Generated with Claude Code