Add 'auto' to PermissionMode type (#785)

## Summary

Adds `"auto"` to the `PermissionMode` Literal type, bringing the Python
SDK to parity with the TypeScript SDK (v0.2.91) and the CLI's
`EXTERNAL_PERMISSION_MODES`.

## Problem

The CLI binary (v2.1.90+) and the TypeScript SDK (v0.2.91) both support
`"auto"` as a valid permission mode, but the Python SDK's
`PermissionMode` type doesn't include it. Users who set
`permission_mode="auto"` get a type error even though the underlying CLI
accepts it.

## Changes

- `src/claude_agent_sdk/types.py`: Added `"auto"` to `PermissionMode =
Literal[...]`
- `tests/test_types.py`: Added test case for `permission_mode="auto"` in
`test_claude_code_options_with_permission_mode`

## Verification

- Confirmed `'auto'` is in `EXTERNAL_PERMISSION_MODES` in the CLI source
(`src/types/permissions.ts`)
- Confirmed TypeScript SDK v0.2.91 already added `'auto'` to its public
`PermissionMode` type
- The Python SDK passes `permission_mode` as `--permission-mode <value>`
to the CLI subprocess with no SDK-side validation — this is purely a
type annotation fix

Author: qing-ant

src/claude_agent_sdk/client.py

@@ -252,6 +252,7 @@ async def set_permission_mode(self, mode: PermissionMode) -> None:
                 - 'plan': Plan-only mode (no tool execution)
                 - 'bypassPermissions': Allow all tools (use with caution)
                 - 'dontAsk': Allow all tools without prompting
+                - 'auto': Automatically determine permission mode
 
         Example:
             ```python

src/claude_agent_sdk/query.py

@@ -58,6 +58,7 @@ async def query(
                  - 'plan': Plan-only mode (no tool execution)
                  - 'bypassPermissions': Allow all tools (use with caution)
                  - 'dontAsk': Allow all tools without prompting
+                 - 'auto': Automatically determine permission mode
                  Set options.cwd for working directory.
         transport: Optional transport implementation. If provided, this will be used
                   instead of the default transport selection based on options.

src/claude_agent_sdk/types.py

@@ -22,7 +22,7 @@
 
 # Permission modes
 PermissionMode = Literal[
-    "default", "acceptEdits", "plan", "bypassPermissions", "dontAsk"
+    "default", "acceptEdits", "plan", "bypassPermissions", "dontAsk", "auto"
 ]
 
 # SDK Beta features - see https://docs.anthropic.com/en/api/beta-headers

tests/test_types.py

@@ -118,6 +118,9 @@ def test_claude_code_options_with_permission_mode(self):
         options_dont_ask = ClaudeAgentOptions(permission_mode="dontAsk")
         assert options_dont_ask.permission_mode == "dontAsk"
 
+        options_auto = ClaudeAgentOptions(permission_mode="auto")
+        assert options_auto.permission_mode == "auto"
+
     def test_claude_code_options_with_system_prompt_string(self):
         """Test Options with system prompt as string."""
         options = ClaudeAgentOptions(