fix(coverage): lax no cover on two matrix-entry-dependent except handlers

Two `except Exception:` handlers fire on some CI matrix entries but not
others — not random, but deterministic per {python, deps, os} combo:

  streamable_http.py:638 — POST handler's outer except. Fires on
  3.12 lowest-direct ubuntu (older dep versions, different shutdown
  exception propagation), not on the other 19 entries.

  streamable_http.py:721 — standalone SSE writer's except. Fires on
  19/20 entries, not on 3.14 locked ubuntu (EventSourceResponse task
  cancellation wins the race there; the closed-stream error wins
  elsewhere).

Both are concurrent-shutdown handlers. The underlying race is
production behavior (client disconnect while server is writing), but
whether tests observe it depends on the interpreter's task scheduling
and the httpx/sse-starlette versions in play. Previously invisible —
subprocess-based tests ran this code in a separate process that
coverage.py couldn't see.

This is the intended use of lax no cover: coverage that differs across
the matrix for reasons outside the test's control.

Author: maxisbey

src/mcp/server/streamable_http.py

@@ -633,7 +633,9 @@ async def sse_writer():  # pragma: lax no cover
                 finally:
                     await sse_stream_reader.aclose()
 
-        except Exception as err:  # pragma: no cover
+        # Fires on some CI matrix entries (3.12 lowest-direct ubuntu) but not
+        # others — concurrent-shutdown exception timing varies by dep version.
+        except Exception as err:  # pragma: lax no cover
             logger.exception("Error handling POST request")
             response = self._create_error_response(
                 f"Error handling POST request: {err}",
@@ -714,7 +716,9 @@ async def standalone_sse_writer():
                         # Send the message via SSE
                         event_data = self._create_event_data(event_message)
                         await sse_stream_writer.send(event_data)
-            except Exception:
+            # Fires on most CI matrix entries but not 3.14 locked ubuntu —
+            # EventSourceResponse cancellation vs stream-close ordering varies.
+            except Exception:  # pragma: lax no cover
                 logger.exception("Error in standalone SSE writer")
             finally:
                 logger.debug("Closing standalone SSE writer")