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>

run_agent.py

@@ -969,6 +969,18 @@ class AIAgent:
         compression_threshold = float(_compression_cfg.get("threshold", 0.50))
         compression_enabled = str(_compression_cfg.get("enabled", True)).lower() in ("true", "1", "yes")
         compression_summary_model = _compression_cfg.get("summary_model") or None
+
+        # Read explicit context_length override from model config
+        _model_cfg = _agent_cfg.get("model", {})
+        if isinstance(_model_cfg, dict):
+            _config_context_length = _model_cfg.get("context_length")
+        else:
+            _config_context_length = None
+        if _config_context_length is not None:
+            try:
+                _config_context_length = int(_config_context_length)
+            except (TypeError, ValueError):
+                _config_context_length = None
         
         self.context_compressor = ContextCompressor(
             model=self.model,
@@ -980,6 +992,7 @@ class AIAgent:
             quiet_mode=self.quiet_mode,
             base_url=self.base_url,
             api_key=getattr(self, "api_key", ""),
+            config_context_length=_config_context_length,
         )
         self.compression_enabled = compression_enabled
         self._user_turn_count = 0