test(gateway): use valid API server key fixture

[embwl0x]

Summary

• update the explicit-disable regression to use a key that meets the production 16-character enrollment gate
• keep the fixture and preservation assertion tied to one local value
• leave production behavior unchanged

Root cause

The API-server enrollment hardening correctly rejects weak API_SERVER_KEY values, but this older regression still used secret-key. The stale fixture made current main fail while asserting that the rejected key was stored.

Validation

• Before: scripts/run_tests.sh tests/gateway/test_config.py — 148 passed, 1 failed
• After: scripts/run_tests.sh tests/gateway/test_config.py — 149 passed
• scripts/run_tests.sh tests/gateway/test_api_server.py tests/gateway/test_platform_connected_checkers.py — 247 passed
• .venv/bin/ruff check tests/gateway/test_config.py
• git diff --check
• publish gate and gitleaks

Fixes #70268

[embwl0x]

Superseded by maintainer PR #70273, which applies the same fixture correction and adds an explanatory strength-gate comment. Closing this duplicate and deferring to the maintainer-owned implementation.

[alt-glitch]

This was generated by AI during triage.

Related to #70268 and #70273: this fixes the same stale API_SERVER_KEY fixture. #70273 is the core-team version with the strength-gate rationale, so this remains related rather than duplicate.