## Why
A persisted login replacement can leave the auth cache holding the previous login, even for the same account. Enterprise MCP authorization must detect that replacement before continuing to use enterprise credentials.
## What changed
- Check cached and persisted account login ownership before enterprise MCP connections and around token exchanges, revoking authorization when ownership changes.
- Recheck account ownership and login generation before committing enterprise sign-in credentials.
- Compare complete token data when checking whether an account OAuth grant is still current, and expose grant token access and credential comparison helpers.
## Testing
Add regression coverage for detecting a same-account login replacement without updating cached auth, rejecting enterprise MCP use after replacement, and keeping ordinary MCP usable while an enterprise account storage read is blocked.
GitOrigin-RevId: 6303f88a9cbcc84c99e453dc281b23d66930f518
jif
2623f206bd