fix(tests): make AIAgent constructor calls self-contained (#11755)

* fix(tests): make AIAgent constructor calls self-contained (no env leakage)

Tests in tests/run_agent/ were constructing AIAgent() without passing
both api_key and base_url, then relying on leaked state from other
tests in the same xdist worker (or process-level env vars) to keep
provider resolution happy. Under hermetic conftest + pytest-split,
that state is gone and the tests fail with 'No LLM provider configured'.

Fix: pass both api_key and base_url explicitly on 47 AIAgent()
construction sites across 13 files. AIAgent.__init__ with both set
takes the direct-construction path (line 960 in run_agent.py) and
skips the resolver entirely.

One call site (test_none_base_url_passed_as_none) left alone — that
test asserts behavior for base_url=None specifically.

This is a prerequisite for any future matrix-split or stricter
isolation work, and lands cleanly on its own.

Validation:
- tests/run_agent/ full: 760 passed, 0 failed (local)
- Previously relied on cross-test pollution; now self-contained

* fix(tests): update opencode-go model order assertion to match kimi-k2.5-first

commit 78a74bb promoted kimi-k2.5 to first position in model suggestion
lists but didn't update this test, which has been failing on main since.
Reorder expected list to match the new canonical order.

tests/run_agent/test_create_openai_client_reuse.py

@@ -23,6 +23,8 @@ from run_agent import AIAgent
 
 def _make_agent():
     return AIAgent(
+        api_key="test-key",
+        base_url="https://openrouter.ai/api/v1",
         model="test/model",
         quiet_mode=True,
         skip_context_files=True,