feat: add background, effort, permissionMode to AgentDefinition (#782)

## Summary

Adds 3 fields to `AgentDefinition` that are present in the TS SDK's
`AgentDefinitionSchema` (`coreSchemas.ts`) but missing from the Python
SDK:

- **`background: bool | None`** -- Run this agent as a background task
(non-blocking, fire-and-forget) when invoked. This is the key addition:
it enables parallel subagent execution without spawning multiple SDK
sessions.
- **`effort: Literal["low", "medium", "high", "max"] | int | None`** --
Reasoning effort level for the agent. Accepts either a named level or an
integer.
- **`permissionMode: PermissionMode | None`** -- Permission mode
controlling how tool executions are handled for this agent.

All three are optional pass-through fields that the CLI already supports
via the `AgentDefinitionSchema` in the control protocol. The existing
`asdict()` + None-filtering serialization in `_internal/client.py`
handles them automatically -- no serialization changes needed.

`permissionMode` reuses the existing `PermissionMode` type alias already
defined in `types.py`.

## Tests

Added 5 new tests in `TestAgentDefinition`:
- `test_background_serializes_correctly` -- verifies boolean
serialization
- `test_effort_accepts_named_level` -- verifies string effort ("high")
- `test_effort_accepts_integer` -- verifies integer effort (32000)
- `test_permission_mode_serializes_as_camelcase` -- verifies camelCase
key
- `test_new_fields_omitted_when_none` -- verifies all 3 fields absent
when unset

All 44 tests pass.

<\!-- CHANGELOG-SDK:START -->
- Added `background`, `effort`, and `permissionMode` fields to
`AgentDefinition` for TS SDK parity
<\!-- CHANGELOG-SDK:END -->

Author: qing-ant

src/claude_agent_sdk/types.py

@@ -82,6 +82,9 @@ class AgentDefinition:
     mcpServers: list[str | dict[str, Any]] | None = None  # noqa: N815
     initialPrompt: str | None = None  # noqa: N815
     maxTurns: int | None = None  # noqa: N815
+    background: bool | None = None
+    effort: Literal["low", "medium", "high", "max"] | int | None = None
+    permissionMode: PermissionMode | None = None  # noqa: N815
 
 
 # Permission Update types (matching TypeScript SDK)

tests/test_types.py

@@ -538,3 +538,64 @@ def test_model_accepts_full_model_id(self):
         payload = self._serialize(agent)
 
         assert payload["model"] == "claude-opus-4-5"
+
+    def test_background_serializes_correctly(self):
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            background=True,
+        )
+        payload = self._serialize(agent)
+
+        assert payload["background"] is True
+
+    def test_effort_accepts_named_level(self):
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            effort="high",
+        )
+        payload = self._serialize(agent)
+
+        assert payload["effort"] == "high"
+
+    def test_effort_accepts_integer(self):
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            effort=32000,
+        )
+        payload = self._serialize(agent)
+
+        assert payload["effort"] == 32000
+
+    def test_permission_mode_serializes_as_camelcase(self):
+        """CLI expects ``permissionMode`` (camelCase)."""
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(
+            description="test",
+            prompt="p",
+            permissionMode="bypassPermissions",
+        )
+        payload = self._serialize(agent)
+
+        assert payload["permissionMode"] == "bypassPermissions"
+        assert "permission_mode" not in payload
+
+    def test_new_fields_omitted_when_none(self):
+        """New optional fields should not appear in payload when unset."""
+        from claude_agent_sdk import AgentDefinition
+
+        agent = AgentDefinition(description="test", prompt="p")
+        payload = self._serialize(agent)
+
+        assert "background" not in payload
+        assert "effort" not in payload
+        assert "permissionMode" not in payload