Add missing age and caughtstealingpercentage fields to SimplePitchingSplit

Fixes #232

The MLB API now includes 'age' and 'caughtstealingpercentage' fields in pitcher statistics responses.
These fields were missing from the SimplePitchingSplit dataclass, causing TypeError when trying to
initialize the object with API response data.

Changes:
- Added age: Optional[int] = None to SimplePitchingSplit
- Added caughtstealingpercentage: Optional[str] = None to SimplePitchingSplit
- Updated docstring to document the new age field

Tested with player ID 641793 which consistently reproduced the issue.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: coreylane

mlbstatsapi/models/stats/pitching.py

@@ -150,6 +150,8 @@ class SimplePitchingSplit:
         The number of inherited runners scored by the pitcher.
     inheritedrunners : int
         The number of inherited runners for the pitcher.
+    age : int
+        The age of the pitcher.
     """
     summary: Optional[str] = None
     gamesplayed: Optional[int] = None
@@ -216,6 +218,8 @@ class SimplePitchingSplit:
     balls: Optional[int] = None
     outspitched: Optional[int] = None
     rbi: Optional[int] = None
+    age: Optional[int] = None
+    caughtstealingpercentage: Optional[str] = None
 
     def __repr__(self) -> str:
         kws = [f'{key}={value}' for key, value in self.__dict__.items() if value is not None and value]