fix: cross-platform test compatibility

- test_channel_plugins: fix assertion logic for discoverable channels
- test_filesystem_tools: normalize path separators for Windows
- test_tool_validation: use python to generate output, avoid cmd line limits

tests/test_tool_validation.py

@@ -379,9 +379,11 @@ async def test_exec_always_returns_exit_code() -> None:
 async def test_exec_head_tail_truncation() -> None:
     """Long output should preserve both head and tail."""
     tool = ExecTool()
-    # Generate output that exceeds _MAX_OUTPUT
-    big = "A" * 6000 + "\n" + "B" * 6000
-    result = await tool.execute(command=f"echo '{big}'")
+    # Generate output that exceeds _MAX_OUTPUT (10_000 chars)
+    # Use python to generate output to avoid command line length limits
+    result = await tool.execute(
+        command="python -c \"print('A' * 6000 + '\\n' + 'B' * 6000)\""
+    )
     assert "chars truncated" in result
     # Head portion should start with As
     assert result.startswith("A")