fix(model_metadata): cache detect_local_server_type result for process lifetime

[uzaylisak]

Problem

Every 5 minutes fetch_endpoint_model_metadata() is called (TTL = _ENDPOINT_MODEL_CACHE_TTL = 300). Each time it calls detect_local_server_type(), which blindly re-runs the full probe waterfall:

1. GET /api/v1/models → 404 (LM Studio check)
2. GET /api/tags → 404 (Ollama check)
3. GET /v1/props → 404 (llama.cpp check)
4. GET /version → 200 (vLLM — found)

On a vllm backend the first three always 404, polluting server logs every 5 minutes. Reported in #29971.

Fix

Add _endpoint_probe_path_cache: Dict[str, str] = {} at module level. detect_local_server_type() checks this cache on entry; on a hit it returns immediately without making any HTTP requests. On the first call the waterfall runs as before, and if a server type is identified it is stored for the lifetime of the process.

The probe stages also use if result is None guards so each step is skipped once an earlier one succeeds — reducing requests even on the very first call.

Impact

• vllm / any server that is not LM Studio or Ollama: only 1 probe round-trip ever, vs. 4 per 5-minute refresh cycle previously.
• LM Studio, Ollama, llama.cpp: one extra dict lookup per call; behaviour is identical.
• Cache is intentionally process-scoped (not TTL-based) because a server type does not change during a session.

Closes #29971.