* Add subject field to AccessToken initialization
Fixes#4266
Add `subject` property to the AccessToken.
```python
return AccessToken(
token=access_token_as_dict["token"],
client_id=access_token_as_dict["client_id"],
scopes=access_token_as_dict["scopes"],
subject=access_token_as_dict["subject"],
# Optional fields
expires_at=access_token_as_dict.get("expires_at"),
resource=access_token_as_dict.get("resource"),
claims=access_token_as_dict.get("claims") or {},
)
```
* Populate AccessToken.subject across all token verifiers
Closes#4266. get_access_token().subject was always None: the SDK's
AccessToken.subject wasn't carried into FastMCP's AccessToken by the
dependency-layer conversion, and none of the built-in TokenVerifiers
(JWT, introspection, and the OAuth-provider verifiers for Discord,
Clerk, Google, WorkOS, HuggingFace, GitHub, and Cognito) populated it
from the sub claim/field they already extract.
---------
Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
