fix: detect context length for custom model endpoints via fuzzy matching + config override (#2051)

* fix: detect context length for custom model endpoints via fuzzy matching + config override

Custom model endpoints (non-OpenRouter, non-known-provider) were silently
falling back to 2M tokens when the model name didn't exactly match what the
endpoint's /v1/models reported. This happened because:

1. Endpoint metadata lookup used exact match only — model name mismatches
   (e.g. 'qwen3.5:9b' vs 'Qwen3.5-9B-Q4_K_M.gguf') caused a miss
2. Single-model servers (common for local inference) required exact name
   match even though only one model was loaded
3. No user escape hatch to manually set context length

Changes:
- Add fuzzy matching for endpoint model metadata: single-model servers
  use the only available model regardless of name; multi-model servers
  try substring matching in both directions
- Add model.context_length config override (highest priority) so users
  can explicitly set their model's context length in config.yaml
- Log an informative message when falling back to 2M probe, telling
  users about the config override option
- Thread config_context_length through ContextCompressor and AIAgent init

Tests: 6 new tests covering fuzzy match, single-model fallback, config
override (including zero/None edge cases).

* fix: auto-detect local model name and context length for local servers

Cherry-picked from PR #2043 by sudoingX.

- Auto-detect model name from local server's /v1/models when only one
  model is loaded (no manual model name config needed)
- Add n_ctx_train and n_ctx to context length detection keys for llama.cpp
- Query llama.cpp /props endpoint for actual allocated context (not just
  training context from GGUF metadata)
- Strip .gguf suffix from display in banner and status bar
- _auto_detect_local_model() in runtime_provider.py for CLI init

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

* fix: revert accidental summary_target_tokens change + add docs for context_length config

- Revert summary_target_tokens from 2500 back to 500 (accidental change
  during patching)
- Add 'Context Length Detection' section to Custom & Self-Hosted docs
  explaining model.context_length config override

---------

Co-authored-by: Test <test@test.com>
Co-authored-by: sudo <sudoingx@users.noreply.github.com>

website/docs/user-guide/configuration.md

@@ -414,6 +414,29 @@ LLM_MODEL=meta-llama/Llama-3.1-70B-Instruct-Turbo
 
 ---
 
+### Context Length Detection
+
+Hermes automatically detects your model's context length by querying the endpoint's `/v1/models` response. For most setups this works out of the box. If detection fails (the model name doesn't match, the endpoint doesn't expose `/v1/models`, etc.), Hermes falls back to a high default and probes downward on context-length errors.
+
+To set the context length explicitly, add `context_length` to your model config:
+
+```yaml
+model:
+  default: "qwen3.5:9b"
+  base_url: "http://localhost:8080/v1"
+  context_length: 131072  # tokens
+```
+
+This takes highest priority — it overrides auto-detection, cached values, and hardcoded defaults.
+
+:::tip When to set this manually
+- Your model shows "2M context" in the status bar (detection failed)
+- You want to limit context below the model's maximum (e.g., 8k on a 128k model to save VRAM)
+- You're running behind a proxy that doesn't expose `/v1/models`
+:::
+
+---
+
 ### Choosing the Right Setup
 
 | Use Case | Recommended |