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/apis/common/type_system.py

@@ -91,15 +91,18 @@ ParamType = register_schema(
     name="ParamType",
 )
 
+"""
 # TODO: recursive definition of ParamType in these containers
 # will cause infinite recursion in OpenAPI generation script
 # since we are going with ChatCompletionInputType and CompletionInputType
 # we don't need to worry about ArrayType/ObjectType/UnionType for now
-# ArrayType.model_rebuild()
-# ObjectType.model_rebuild()
-# UnionType.model_rebuild()
+ArrayType.model_rebuild()
+ObjectType.model_rebuild()
+UnionType.model_rebuild()
 
 
-# class CustomType(BaseModel):
-#     type: Literal["custom"] = "custom"
-#     validator_class: str
+class CustomType(BaseModel):
+pylint: disable=syntax-error
+    type: Literal["custom"] = "custom"
+    validator_class: str
+"""