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>

agent/context_compressor.py

@@ -46,6 +46,7 @@ class ContextCompressor:
         summary_model_override: str = None,
         base_url: str = "",
         api_key: str = "",
+        config_context_length: int | None = None,
     ):
         self.model = model
         self.base_url = base_url
@@ -56,7 +57,10 @@ class ContextCompressor:
         self.summary_target_tokens = summary_target_tokens
         self.quiet_mode = quiet_mode
 
-        self.context_length = get_model_context_length(model, base_url=base_url, api_key=api_key)
+        self.context_length = get_model_context_length(
+            model, base_url=base_url, api_key=api_key,
+            config_context_length=config_context_length,
+        )
         self.threshold_tokens = int(self.context_length * threshold_percent)
         self.compression_count = 0
         self._context_probed = False  # True after a step-down from context error