fix(agent): route Anthropic stream cleanup through _anthropic_client, not OpenAI rebuild

[liuhao1024]

What does this PR do?

When api_mode == "anthropic_messages", three streaming-cleanup paths in agent/chat_completion_helpers.py unconditionally called _replace_primary_openai_client() to rebuild the connection pool after stream errors. For Anthropic-only configurations this:

1. Failed with Missing credentials. Please pass an api_key because OPENAI_API_KEY is unset
2. Left the Anthropic httpx stream open — the worker thread blocked on the dead socket until the 900s read-timeout fired

User-visible symptom: the agent appears to hang ~15 minutes on stuck streams before any retry or fallback engages.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

File	Change
agent/chat_completion_helpers.py	Route 3 cleanup paths through Anthropic client when api_mode == "anthropic_messages"
tests/run_agent/test_streaming.py	Update existing retry test assertions + add regression test for stale-stream cleanup

How to Test

1. Run pytest tests/ -q — all tests should pass
2. Verify the specific scenario described above is resolved

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 26.4.1

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture and workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A

Code Intelligence

• Analyzed: agent/chat_completion_helpers.py:interruptible_streaming_api_call (streaming cleanup paths)
• Blast radius: LOW — change is guarded by api_mode check, OpenAI path unchanged
• Related patterns: existing _interrupt_requested handler at lines ~2090-2092 already uses the same Anthropic cleanup pattern

[alt-glitch]

Competes with #28240 and #33855 — all fix the same 3 streaming-cleanup paths routing through OpenAI client instead of Anthropic (#28161). This PR and #28240 cover the same scope.

[teknium1]

Closed in favor of #53926, now merged to main (commit a0b9663).

This was a popular bug — 12 independent PRs fixed the same issue (#28161): the three stream-cleanup paths in the streaming code rebuilt the OpenAI primary client unconditionally, which on Anthropic-native sessions both failed (no OPENAI_API_KEY) and left the wedged stream open, causing the ~15-minute hang.

We salvaged @EloquentBrush0x's #28240 (the earliest dedicated fix for this issue) onto current main, resolved the conflict against the newer request-client cleanup helper, repointed the bug-encoding regression test, and added coverage for the two reachable Anthropic cleanup sites. Verified live: stale stream torn down in <1s instead of ~900s, Anthropic client closed+rebuilt, OpenAI rebuild never called on the Anthropic path.

Thank you for the fix — closing as a duplicate of the merged work. Credit to everyone who reported and fixed this.

#53926