Add no branch pragmas to phase-1 ClientSession blocks

The block-exit arcs from these three async with lines were never
actually traced on 3.11 or 3.14 — on main, the arc destinations were
lax-no-cover lines, so coverage silently excluded the arcs from
counting. Removing those destinations in the previous commit exposed
the latent gap.

Adding no branch here is a smaller suppression than what we removed:
the assertions that used to be lax-no-cover are now fully verified,
and we only suppress branch-exit tracking on async with lines that
have no actual branching. Matches the existing no branch on the
phase-2 ClientSession lines in these same tests.

Author: maxisbey

tests/shared/test_streamable_http.py

@@ -1132,7 +1132,7 @@ async def test_streamable_http_client_session_termination(basic_server: None, ba
             read_stream,
             write_stream,
         ):
-            async with ClientSession(read_stream, write_stream) as session:
+            async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
                 # Initialize the session
                 result = await session.initialize()
                 assert isinstance(result, InitializeResult)
@@ -1193,7 +1193,7 @@ async def mock_delete(self: httpx.AsyncClient, *args: Any, **kwargs: Any) -> htt
             read_stream,
             write_stream,
         ):
-            async with ClientSession(read_stream, write_stream) as session:
+            async with ClientSession(read_stream, write_stream) as session:  # pragma: no branch
                 # Initialize the session
                 result = await session.initialize()
                 assert isinstance(result, InitializeResult)
@@ -1251,7 +1251,9 @@ async def on_resumption_token_update(token: str) -> None:
             read_stream,
             write_stream,
         ):
-            async with ClientSession(read_stream, write_stream, message_handler=message_handler) as session:
+            async with ClientSession(  # pragma: no branch
+                read_stream, write_stream, message_handler=message_handler
+            ) as session:
                 # Initialize the session
                 result = await session.initialize()
                 assert isinstance(result, InitializeResult)