chore: ruff auto-fix PLR6201 resweep — tuple → set in membership tests (#27355)

Six days after #23937 (608 fixes) the codebase had accumulated 241 new
PLR6201 violations. Same mechanical `x in (...)` → `x in {...}` fix,
same zero-risk profile: set lookup is O(1) vs O(n) for tuple and the
two are semantically equivalent for hashable scalar membership tests.

All 241 instances fixed via `ruff check --select PLR6201 --fix
--unsafe-fixes`, zero remaining. Every changed value is a hashable
scalar (str/int/None/enum/signal); no risk of unhashable runtime
errors. No behavior change.

Test plan:
- 119 files changed, +244/-244 (net zero) — exactly one-line edits
- `ruff check` clean afterward
- Compile checks pass on the largest touched files (cli.py, run_agent.py,
  gateway/run.py, gateway/platforms/discord.py, model_tools.py)
- Subset broad test run on tests/gateway/ tests/hermes_cli/ tests/agent/
  tests/tools/: 18187 passed, 59 pre-existing failures (verified against
  origin/main with the same shape — identical failure count, identical
  category — all xdist test-order flakes unrelated to this change)

Follows the same template as PR #23937 ([tracker: #23972](https://github.com/NousResearch/hermes-agent/issues/23972)).

tests/tools/test_code_execution_modes.py

@@ -125,7 +125,7 @@ class TestResolveChildPython(unittest.TestCase):
     def test_project_with_no_venv_falls_back(self):
         """Project mode without VIRTUAL_ENV or CONDA_PREFIX → sys.executable."""
         env = {k: v for k, v in os.environ.items()
-               if k not in ("VIRTUAL_ENV", "CONDA_PREFIX")}
+               if k not in {"VIRTUAL_ENV", "CONDA_PREFIX"}}
         with patch.dict(os.environ, env, clear=True):
             self.assertEqual(_resolve_child_python("project"), sys.executable)