fix: warn on invalid context_length format in config.yaml (#10067)

Previously, non-integer context_length values (e.g. '256K') in
config.yaml were silently ignored, causing the agent to fall back
to 128K auto-detection with no user feedback. This was confusing
for users with custom LiteLLM endpoints expecting larger context.

Now prints a clear stderr warning and logs at WARNING level when
model.context_length or custom_providers[].models.<model>.context_length
cannot be parsed as an integer, telling users to use plain integers
(e.g. 256000 instead of '256K').

Reported by community user ChFarhan via Discord.

tests/run_agent/test_invalid_context_length_warning.py

@@ -0,0 +1,111 @@
+"""Tests that invalid context_length values in config produce visible warnings."""
+
+from unittest.mock import patch, MagicMock, call
+
+
+def _build_agent(model_cfg, custom_providers=None, model="anthropic/claude-opus-4.6"):
+    """Build an AIAgent with the given model config."""
+    cfg = {"model": model_cfg}
+    if custom_providers is not None:
+        cfg["custom_providers"] = custom_providers
+
+    with (
+        patch("hermes_cli.config.load_config", return_value=cfg),
+        patch("agent.model_metadata.get_model_context_length", return_value=128_000),
+        patch("run_agent.get_tool_definitions", return_value=[]),
+        patch("run_agent.check_toolset_requirements", return_value={}),
+        patch("run_agent.OpenAI"),
+    ):
+        from run_agent import AIAgent
+
+        agent = AIAgent(
+            model=model,
+            api_key="test-key-1234567890",
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+        )
+    return agent
+
+
+def test_valid_integer_context_length_no_warning():
+    """Plain integer context_length should work silently."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": 256000})
+    assert agent._config_context_length == 256000
+    # No warning about invalid context_length
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)
+
+
+def test_string_k_suffix_context_length_warns():
+    """context_length: '256K' should warn the user clearly."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": "256K"})
+    assert agent._config_context_length is None
+    # Should have warned
+    warning_calls = [c for c in mock_logger.warning.call_args_list
+                     if "Invalid" in str(c) and "256K" in str(c)]
+    assert len(warning_calls) == 1
+    assert "plain integer" in str(warning_calls[0])
+
+
+def test_string_numeric_context_length_works():
+    """context_length: '256000' (string) should parse fine via int()."""
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent({"default": "gpt5.4", "provider": "custom",
+                              "base_url": "http://localhost:4000/v1",
+                              "context_length": "256000"})
+    assert agent._config_context_length == 256000
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)
+
+
+def test_custom_providers_invalid_context_length_warns():
+    """Invalid context_length in custom_providers should warn."""
+    custom_providers = [
+        {
+            "name": "LiteLLM",
+            "base_url": "http://localhost:4000/v1",
+            "models": {
+                "gpt5.4": {"context_length": "256K"}
+            },
+        }
+    ]
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent(
+            {"default": "gpt5.4", "provider": "custom",
+             "base_url": "http://localhost:4000/v1"},
+            custom_providers=custom_providers,
+            model="gpt5.4",
+        )
+    warning_calls = [c for c in mock_logger.warning.call_args_list
+                     if "Invalid" in str(c) and "256K" in str(c)]
+    assert len(warning_calls) == 1
+    assert "custom_providers" in str(warning_calls[0])
+
+
+def test_custom_providers_valid_context_length():
+    """Valid integer in custom_providers should work silently."""
+    custom_providers = [
+        {
+            "name": "LiteLLM",
+            "base_url": "http://localhost:4000/v1",
+            "models": {
+                "gpt5.4": {"context_length": 256000}
+            },
+        }
+    ]
+    with patch("run_agent.logger") as mock_logger:
+        agent = _build_agent(
+            {"default": "gpt5.4", "provider": "custom",
+             "base_url": "http://localhost:4000/v1"},
+            custom_providers=custom_providers,
+            model="gpt5.4",
+        )
+    for c in mock_logger.warning.call_args_list:
+        assert "Invalid" not in str(c)