Narrow settings.model at service boundaries, not via truthiness

Two services were reading `_settings.model` (typed `str | _NotGiven |
None` because NOT_GIVEN is the default) and coercing it with `or ""`
or similar. `_NotGiven.__bool__` returns False, so the runtime
behavior happened to work, but the type was a lie — pyright saw
`str | _NotGiven` flowing into APIs that required `str` or `str | None`.

- `AIService._sync_model_name_to_metrics`: use `isinstance(model, str)`
  narrowing with an empty-string fallback. Equivalent runtime behavior,
  honest type, no truthiness dependency on a sentinel.
- `SarvamLLMService.__init__`: validate the model is a real string
  before handing it to `_validate_model(str)`. A non-string model at
  this point is a configuration bug; raise `ValueError` so the error
  is clear and survives `python -O` (unlike an assert).

pyrightconfig.json

@@ -2,14 +2,8 @@
   "typeCheckingMode": "basic",
   "pythonVersion": "3.11",
   "pythonPlatform": "All",
-  "include": [
-    "scripts",
-    "src/pipecat"
-  ],
-  "exclude": [
-    "**/*_pb2.py",
-    "**/__pycache__"
-  ],
+  "include": ["scripts", "src/pipecat"],
+  "exclude": ["**/*_pb2.py", "**/__pycache__"],
   "ignore": [
     "tests",
     "src/pipecat/adapters/base_llm_adapter.py",
@@ -40,7 +34,6 @@
     "src/pipecat/processors/frameworks/rtvi/processor.py",
     "src/pipecat/processors/frameworks/strands_agents.py",
     "src/pipecat/processors/gstreamer/pipeline_source.py",
-    "src/pipecat/services/ai_service.py",
     "src/pipecat/services/anthropic/llm.py",
     "src/pipecat/services/assemblyai/stt.py",
     "src/pipecat/services/asyncai/tts.py",
@@ -106,7 +99,6 @@
     "src/pipecat/services/resembleai/tts.py",
     "src/pipecat/services/rime/tts.py",
     "src/pipecat/services/sambanova/llm.py",
-    "src/pipecat/services/sarvam/llm.py",
     "src/pipecat/services/sarvam/stt.py",
     "src/pipecat/services/sarvam/tts.py",
     "src/pipecat/services/simli/video.py",
@@ -138,4 +130,4 @@
     "src/pipecat/transports/whatsapp/client.py"
   ],
   "reportMissingImports": false
-}
+}