fix(tests): pass base_url to 31 more AIAgent() calls across run_agent tests

Same root cause as previous commit — tests that construct AIAgent()
without base_url rely on provider-resolver fallback state that doesn't
exist in hermetic CI / shard-split runs. Previously hidden because
other tests in the same xdist worker happened to prime module state.

Covered by the previous fix: calls that passed api_key but not base_url.
This covers calls that pass NEITHER (model=... only) — test_streaming.py
especially (24 call sites). Plus test_860_dedup, test_compression_
persistence, test_create_openai_client_*, test_provider_parity.

One call site (test_none_base_url_passed_as_none) remains explicitly
unmodified — it asserts None/empty base_url behavior, so adding base_url
would defeat the test's intent.

Validation:
- tests/run_agent/: 760 passed, 0 failed (local)
- Matrix shard 3 subset: 3098 passed, 0 failed, 1m49s (local)

tests/run_agent/test_create_openai_client_reuse.py

@@ -23,6 +23,7 @@ from run_agent import AIAgent
 
 def _make_agent():
     return AIAgent(
+        base_url="https://openrouter.ai/api/v1",
         model="test/model",
         quiet_mode=True,
         skip_context_files=True,