fix: use default_factory for mutable field defaults in ServerCallContext (a2aproject#744)

## summary
- replace `default={}` and `default=UnauthenticatedUser()` with
`default_factory=dict` and `default_factory=UnauthenticatedUser` in
`ServerCallContext` to prevent shared mutable default instances across
all instances.

Author: greysonlalonde

src/a2a/server/context.py

@@ -19,7 +19,7 @@ class ServerCallContext(BaseModel):
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
-    state: State = Field(default={})
-    user: User = Field(default=UnauthenticatedUser())
+    state: State = Field(default_factory=dict)
+    user: User = Field(default_factory=UnauthenticatedUser)
     requested_extensions: set[str] = Field(default_factory=set)
     activated_extensions: set[str] = Field(default_factory=set)