The check flagged an untested backend behavior change, and it was right —
withholding the MCP credential is the whole feature, and nothing asserted it.

Credential issuance is the observable that matters: it is the only place a
credential is minted and `/mcp` accepts nothing else, so withholding it is
what actually denies every provider and any external MCP client.

Testing it needed a seam. The real issuer reads a module-global registry that
only a running MCP server installs, so from a unit test the enabled and
disabled paths look identical — both leave no session behind. Reading the
stored session back would have passed either way, which is worse than no test.
`ProviderServiceLiveOptions` already carries a test override for the canonical
event logger, so the issuer follows that existing pattern.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>