test: replace if/else branch with single assert to satisfy 100% branch coverage

The prior if/else on sys.version_info created a branch where only one
side executes per CI job (each job pins a single Python version), so
coverage's --cov-branch --cov-fail-under=100 flags the always-taken
branch as partial on every job in the matrix (all failed on this PR's
last CI run). Collapsing to a single assert against a version-gated
tuple keeps the exact same behavioral pinning (PARSE_ERROR only below
3.14, either code on 3.14+) without introducing an untestable branch.

Verified locally on Python 3.10 (lowest supported): full suite passes
(5444 passed, 9 skipped, 1 xfailed), coverage report shows 100.00%
total including this file, ruff check and ruff format --check clean.

<sub>AI Disclosure: https://gist.github.com/maxisbey/6123d132484e4c533eab519a2800693d</sub>

Author: g0rdonL

tests/server/test_streamable_http_modern.py

@@ -1030,10 +1030,8 @@ async def test_modern_post_with_deeply_nested_body_is_rejected_not_a_crash() ->
     async with _asgi_client(_x_mcp_server()) as http:
         response = await http.post("/mcp", content=body, headers={"content-type": "application/json"})
     assert response.status_code == 400
-    if sys.version_info >= (3, 14):
-        assert response.json()["error"]["code"] in (PARSE_ERROR, INVALID_REQUEST)
-    else:
-        assert response.json()["error"]["code"] == PARSE_ERROR
+    allowed_codes = (PARSE_ERROR, INVALID_REQUEST) if sys.version_info >= (3, 14) else (PARSE_ERROR,)
+    assert response.json()["error"]["code"] in allowed_codes
 
 
 async def test_modern_post_recursion_error_during_parse_is_parse_error(monkeypatch: pytest.MonkeyPatch) -> None: