fix(api)!: authentication method type is an enum, not a plain string (#442)

# Migration
`AuthenticationMethod.type` was incorrectly defined as a string. Instead the API defines it as a string enum with values `assisted, credential, api_token, api_credential, oauth`

Author: stainless-app[bot]

.stats.yml

@@ -1,2 +1,2 @@
 configured_endpoints: 37
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/finch-6c0cb9620225dbe9e7180b514e51b2cd94753565749de1603e77d065cb531be1.yml
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/finch-aeb62bc4875f4eb1b2ae1f75ef9521e92022e863cba2ae67f907ed27ca93d527.yml

src/finch/types/introspection.py

@@ -18,7 +18,8 @@ class AuthenticationMethodConnectionStatus(BaseModel):
 class AuthenticationMethod(BaseModel):
     connection_status: Optional[AuthenticationMethodConnectionStatus] = None
 
-    type: Optional[str] = None
+    type: Optional[Literal["assisted", "credential", "api_token", "api_credential", "oauth"]] = None
+    """The type of authentication method."""
 
 
 class Introspection(BaseModel):