fix: use lax no cover for non-deterministic coverage paths

These validation functions and exception handlers have coverage that
varies between parallel test runs. Using 'lax no cover' correctly
excludes them from coverage measurement without triggering
strict-no-cover violations when they happen to be covered.

Github-Issue:#1648

Author: skyvanguard

src/mcp/server/streamable_http.py

@@ -806,14 +806,14 @@ async def _handle_unsupported_request(self, request: Request, send: Send) -> Non
         )
         await response(request.scope, request.receive, send)
 
-    async def _validate_request_headers(self, request: Request, send: Send) -> bool:
+    async def _validate_request_headers(self, request: Request, send: Send) -> bool:  # pragma: lax no cover
         if not await self._validate_session(request, send):
             return False
         if not await self._validate_protocol_version(request, send):
             return False
         return True
 
-    async def _validate_session(self, request: Request, send: Send) -> bool:
+    async def _validate_session(self, request: Request, send: Send) -> bool:  # pragma: lax no cover
         """Validate the session ID in the request."""
         if not self.mcp_session_id:
             # If we're not using session IDs, return True
@@ -842,7 +842,7 @@ async def _validate_session(self, request: Request, send: Send) -> bool:
 
         return True
 
-    async def _validate_protocol_version(self, request: Request, send: Send) -> bool:
+    async def _validate_protocol_version(self, request: Request, send: Send) -> bool:  # pragma: lax no cover
         """Validate the protocol version header in the request."""
         # Get the protocol version from the request headers
         protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER)

src/mcp/server/streamable_http_manager.py

@@ -182,7 +182,7 @@ async def run_stateless_server(*, task_status: TaskStatus[None] = anyio.TASK_STA
                         self.app.create_initialization_options(),
                         stateless=True,
                     )
-                except Exception:
+                except Exception:  # pragma: lax no cover
                     logger.exception("Stateless session crashed")
 
         # Assert task group is not None for type checking