#13940: fix: suppress low context window warning for explicitly configured models

## Problem When a model's `contextWindow` is explicitly set in `openclaw.json` (e.g., `2048` for a cheap heartbeat model), OpenClaw still logs: ``` warn agent/embedded low context window: ollama/qwen2.5:7b-2k ctx=2048 (warn<32000) source=modelsConfig ``` This is noise — the user deliberately chose that value. ## Fix Skip the warning when `source === "modelsConfig"` (user explicitly configured it). The warning still fires for auto-detected (`model` metadata) and `default` sources where the value might be wrong. `shouldBlock` is **unaffected** — the hard minimum is still enforced regardless of source, so genuinely broken configs are still caught. ## Changes - `src/agents/context-window-guard.ts`: Added `source !== "modelsConfig"` condition to `shouldWarn` - `src/agents/context-window-guard.test.ts`: Added test case for explicit modelsConfig with small context window Closes #13933 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Suppresses low context window warnings when `contextWindow` is explicitly configured in `openclaw.json` via `models.providers.<provider>.models[].contextWindow`. The warning still fires for auto-detected (`model` metadata) and `default` sources where the value might be incorrect. - Modified `shouldWarn` logic to skip warnings when `source === "modelsConfig"` - Added test case validating the new behavior - `shouldBlock` remains unchanged, preserving hard minimum enforcement <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is focused and well-tested with a new test case that validates the fix. The logic correctly suppresses warnings only for explicitly configured models while preserving the hard minimum blocking behavior and warnings for other sources. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->