ci: add mypy for static type checking (#1101)

# What does this PR do?

- Enable mypy to run in the CI on a subset of the repository
- Fix a few mypy errors
- Run mypy from pre-commit

Signed-off-by: Sébastien Han <seb@redhat.com>
 
[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/schema_utils.py

@@ -5,12 +5,10 @@
 # the root directory of this source tree.
 
 from dataclasses import dataclass
-from typing import Any, Callable, List, Optional, TypeVar
+from typing import Any, Callable, List, Optional, Protocol, TypeVar
 
 from .strong_typing.schema import json_schema_type, register_schema  # noqa: F401
 
-T = TypeVar("T")
-
 
 @dataclass
 class WebMethod:
@@ -22,6 +20,13 @@ class WebMethod:
     raw_bytes_request_body: Optional[bool] = False
 
 
+class HasWebMethod(Protocol):
+    __webmethod__: WebMethod
+
+
+T = TypeVar("T", bound=HasWebMethod)  # Bound T to classes that match this protocol
+
+
 def webmethod(
     route: Optional[str] = None,
     method: Optional[str] = None,