fix(concurrency): remove TextIOBase inheritance to avoid property override issue

- _ThreadLocalStream no longer inherits from io.TextIOBase
- Implements all required stream methods directly (write, flush, encoding, etc.)
- Removes need for type: ignore[override] on encoding property
- Fixes Pyright type checking error on Python 3.13
- Maintains full compatibility with sys.stdout/sys.stderr assignment

src/cleveragents/application/services/validation_pipeline.py

@@ -45,7 +45,7 @@ logger = logging.getLogger(__name__)
 # ---------------------------------------------------------------------------
 
 
-class _ThreadLocalStream(io.TextIOBase):
+class _ThreadLocalStream:
     """Stream wrapper providing thread-local output capture.
 
     Non-capturing threads write through to the original stream.
@@ -59,7 +59,7 @@ class _ThreadLocalStream(io.TextIOBase):
         self._local = threading.local()
 
     @property
-    def encoding(self) -> str | None:  # matches io.TextIOBase.encoding signature
+    def encoding(self) -> str:  # matches io.TextIOBase.encoding signature
         return getattr(self._original, "encoding", "utf-8")
 
     def writable(self) -> bool: