Fix uv pip install: use venv dir not python binary path, add .exe on Windows

- uv pip --python works better with the venv directory path than the
  python binary path (avoids "No virtual environment found" on Windows)
- Add .exe suffix to Windows python path for non-uv fallback

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

python/build_sidecar.py

@@ -92,14 +92,18 @@ def create_venv_and_install(cpu_only: bool) -> Path:
 
     # Determine python path inside venv
     if sys.platform == "win32":
-        python = str(venv_dir / "Scripts" / "python")
+        python = str(venv_dir / "Scripts" / "python.exe")
     else:
         python = str(venv_dir / "bin" / "python")
 
     def pip_install(*args: str) -> None:
         """Install packages. Pass package names and flags only, not 'install'."""
         if use_uv:
-            subprocess.run(["uv", "pip", "install", "--python", python, *args], check=True)
+            # Use --python with the venv directory (not the python binary) for uv
+            subprocess.run(
+                ["uv", "pip", "install", "--python", str(venv_dir), *args],
+                check=True,
+            )
         else:
             subprocess.run([python, "-m", "pip", "install", *args], check=True)