fix(cli): Fix TypeAdapter NameError when pydantic is not installed

[Mr-Neutr0n]

What does this PR do?

Fixes a NameError: name 'TypeAdapter' is not defined error when importing transformers without pydantic installed.

Problem

The TypeAdapter class from pydantic was used as a type annotation in _validate_request() at line 588:

def _validate_request(
    self,
    request: dict,
    schema: TypedDict,
    validator: TypeAdapter,  # <-- NameError here
    unused_fields: set,
):

However, TypeAdapter is only imported inside an if serve_dependencies_available: block, which requires pydantic to be installed. When pydantic is not installed, Python raises NameError when parsing the class definition.

Solution

1. Added TypeAdapter to the TYPE_CHECKING imports block
2. Changed the type annotation to use a string literal "TypeAdapter" for forward reference

This ensures the type annotation is only evaluated during static type checking, not at runtime.

Fixes #43824