feat: allow custom endpoints to use responses API via api_mode override (#1651)

Add HERMES_API_MODE env var and model.api_mode config field to let
custom OpenAI-compatible endpoints opt into codex_responses mode
without requiring the OpenAI Codex OAuth provider path.

- _get_configured_api_mode() reads HERMES_API_MODE env (precedence)
  then model.api_mode from config.yaml; validates against whitelist
- Applied in both _resolve_openrouter_runtime() and
  _resolve_named_custom_runtime() (original PR only covered openrouter)
- Fix _dump_api_request_debug() to show /responses URL when in
  codex_responses mode instead of always showing /chat/completions
- Tests for config override, env override, invalid values, named
  custom providers, and debug dump URL for both API modes

Inspired by PR #1041 by @mxyhi.

Co-authored-by: mxyhi <mxyhi@users.noreply.github.com>

tests/test_run_agent_codex_responses.py

@@ -750,3 +750,40 @@ def test_run_conversation_codex_continues_after_ack_for_directory_listing_prompt
         for msg in result["messages"]
     )
     assert any(msg.get("role") == "tool" and msg.get("tool_call_id") == "call_1" for msg in result["messages"])
+
+
+def test_dump_api_request_debug_uses_responses_url(monkeypatch, tmp_path):
+    """Debug dumps should show /responses URL when in codex_responses mode."""
+    import json
+    agent = _build_agent(monkeypatch)
+    agent.base_url = "http://127.0.0.1:9208/v1"
+    agent.logs_dir = tmp_path
+
+    dump_file = agent._dump_api_request_debug(_codex_request_kwargs(), reason="preflight")
+
+    payload = json.loads(dump_file.read_text())
+    assert payload["request"]["url"] == "http://127.0.0.1:9208/v1/responses"
+
+
+def test_dump_api_request_debug_uses_chat_completions_url(monkeypatch, tmp_path):
+    """Debug dumps should show /chat/completions URL for chat_completions mode."""
+    import json
+    _patch_agent_bootstrap(monkeypatch)
+    agent = run_agent.AIAgent(
+        model="gpt-4o",
+        base_url="http://127.0.0.1:9208/v1",
+        api_key="test-key",
+        quiet_mode=True,
+        max_iterations=1,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+    agent.logs_dir = tmp_path
+
+    dump_file = agent._dump_api_request_debug(
+        {"model": "gpt-4o", "messages": [{"role": "user", "content": "hi"}]},
+        reason="preflight",
+    )
+
+    payload = json.loads(dump_file.read_text())
+    assert payload["request"]["url"] == "http://127.0.0.1:9208/v1/chat/completions"