fix(transports): use PEP 604 annotation for ToolCall.extra_content

`ToolCall.extra_content` was annotated `Optional[Dict[str, Any]]`,
but neither `Optional` nor `Dict` are imported at the top of
`agent/transports/types.py` — only `Any` is.  The rest of the file
consistently uses PEP 604 / 585 syntax (e.g. `str | None`,
`dict[str, Any] | None`).

The file has `from __future__ import annotations`, so the missing
names don't crash class definition.  But the annotation IS evaluated
when anything calls `typing.get_type_hints(ToolCall)` —
introspection raises `NameError: name 'Optional' is not defined`.

ruff catches it cleanly:

    F821 Undefined name `Optional`  agent/transports/types.py:65:32
    F821 Undefined name `Dict`      agent/transports/types.py:65:41

Switch the annotation to `dict[str, Any] | None` to match the
rest of the file's style.  No new imports needed.

Verified:
  - ruff F-checks now pass on the file
  - `typing.get_type_hints(ToolCall)` succeeds where it raised before
  - 166/166 tests in tests/agent/transports/ pass on Windows + Python 3.12

agent/transports/types.py

@@ -62,7 +62,7 @@ class ToolCall:
         return (self.provider_data or {}).get("response_item_id")
 
     @property
-    def extra_content(self) -> Optional[Dict[str, Any]]:
+    def extra_content(self) -> dict[str, Any] | None:
         """Gemini extra_content (thought_signature) from provider_data.
 
         Gemini 3 thinking models attach ``extra_content`` with a