test: keep gateway-dependent e2e out of the commit gate

The pre-commit hook runs the whole suite, and tests/e2e/ talks to a real
LLM gateway, so whether a commit is allowed depended on how fast that
gateway happened to be. During the issue 14 work it blocked two commits
on two different cases; both passed when rerun alone, and the suite went
from 165s to 336s that hour.

The wasted minutes are not the real cost. Retrying on red teaches you to
read "test failed" as "gateway was slow", and a genuinely flaky bug then
gets retried away too. An alarm that cries wolf stops being an alarm.

test_thinking_live.py already carried the slow marker; the other three
files now match it, and the release checklist gains an explicit
`pytest -m slow` step so they still run where a human is watching --
without that step this change would just delete the coverage.

Also raises test_flat_legacy_keys_assemble's LLM_TIMEOUT from 120 to
300, matching .env. At 120 the case allowed half of what production
allows, on a gateway that needs the full 300 -- it measured 116s in a
solo run. The assertion is that the flat key name parses into
SourceConfig.timeout_s; the value itself was never under test.

tests/e2e/test_embed_probe.py

@@ -21,9 +21,15 @@ from polygateway.types import SourceConfig
 
 _ENV = {k: v for k, v in {**dotenv_values(".env"), **os.environ}.items() if v is not None}
 
-pytestmark = pytest.mark.skipif(
-    "LLM__MINIMAX__1__BASE_URL" not in _ENV, reason="缺真实网关配置(.env)"
-)
+# 真实网关调用: 与 test_thinking_live.py 同待遇标 slow(pytest addopts 默认排除,
+# 显式 `pytest -m slow` 运行)。理由见 test_compat_projects.py 同处注释。
+pytestmark = [
+    pytest.mark.slow,
+    pytest.mark.skipif(
+        "LLM__MINIMAX__1__BASE_URL" not in _ENV,
+        reason="缺真实网关配置(.env)",
+    ),
+]
 
 _OUT = Path("tests/outputs/embedding")