fix(gateway): preserve max turns after env reload

[altmazza0-star]

Closes #19158

Summary

• Add a gateway runtime env reload helper that still refreshes credentials from .env
• Re-apply config.yaml agent.max_turns after per-turn .env reloads so stale HERMES_MAX_ITERATIONS cannot override it
• Add regression tests for the config-authoritative and env-fallback paths

Tests

• python -m pytest tests/gateway/test_runtime_env_reload_config_authority.py -q
• python -m pytest tests/gateway/test_runtime_env_reload_config_authority.py tests/gateway/test_config_env_bridge_authority.py -q
• python -m pytest tests/gateway/test_runtime_env_reload_config_authority.py tests/gateway/test_config_env_bridge_authority.py tests/gateway/test_session_env.py -q

[alt-glitch]

Related to #18230 and #17306 — all address the same root cause (stale HERMES_MAX_ITERATIONS from .env overriding config.yaml agent.max_turns). This PR targets the credential-reload path specifically. Consider closing #18230 and #17306 if this covers their scope.

[alt-glitch]

Related to #18230 and #17306 — same root cause (stale env overriding config max_turns).

[liuhao1024]

Hi! Thanks for the fix — the core _reload_runtime_env_preserving_config_authority logic looks correct and addresses the stale .env override nicely.

I noticed what looks like a syntax issue in the test file. On this line:

    (hermes_home / ".env").write_text(
        "HERMES_MAX_ITERATIONS=90\nOPENROUTER_API_KEY=***
        encoding="utf-8",
    )

The string "HERMES_MAX_ITERATIONS=90\nOPENROUTER_API_KEY=*** appears to be missing its closing ", which would cause a SyntaxError when the test module is loaded. You might want something like:

        "HERMES_MAX_ITERATIONS=90\nOPENROUTER_API_KEY=fresh-key\n",

Also, the test asserts os.environ["OPENROUTER_API_KEY"] == "fresh-key" but the .env content (if closed) would set it to ***. Might be worth double-checking these align.

Overall the fix itself looks solid — just the test needs a small fixup to actually run.