fix: re-login and re-send when phpMyAdmin expires the session

[blaipr]

What this changes

PhpMyAdminClient.runOne checks each response for the login form (input[name="pma_username"]) before reading it as a result. When it appears, the client clears the session, logs in again and re-sends the statement once; if the login form comes back a second time the statement fails by name (StatementError) instead of being reported as applied. login() now returns the token, and clears stale cookies before starting a new session. docs/phpmyadmin.md records the new selector and the behaviour.

Why

phpMyAdmin drops an idle session after 24 minutes by default and answers the next POST with its login page — status 200, no error, no grid, no affected count, and a token input of its own. An MCP server lives far longer than that, so the first db_execute of the afternoon met this page and was reported as applied with nothing affected, and the login form's token was quietly adopted as the rotated one. Nothing ran — an unauthenticated POST is turned away before it reaches MySQL — so the single retry cannot apply a statement twice. Covered by tests over a scripted fetch; sabotage-checked against the previous client (2 fail).

Checklist

• npm test passes
• npm run typecheck passes
• Documentation in docs/ updated if behaviour changed
• CHANGELOG.md updated if the change is user-observable
• No credentials, hostnames or database names in the diff