`ChromiumCookieReadError` carried only a reason and a cause, so every
`readFailed` and keychain refusal logged identically. A user with several
Chromium browsers installed had no way to tell which one refused. The database
path is now a structural attribute and the message derives from it, matching
how `BrowserSession`'s errors carry their partition.

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