Fix grouped database token invalidation

[pedrocarlo]

Summary

• Route turso db tokens invalidate <database> to the database-scoped rotation endpoint for grouped as well as ungrouped databases, rather than refusing grouped databases or substituting group rotation.
• Correct the database confirmation: database-scoped tokens need replacement; group tokens are unaffected. Do not claim replica restarts or downtime.
• Clarify turso group tokens invalidate help, confirmation, and success output: this invalidates group tokens but not necessarily database-scoped tokens. To revoke the latter, run turso db tokens invalidate <database-name> for each database. Avoid claiming all database connections drop or that downtime occurs.
• Add command-level and endpoint tests for grouped databases and the legacy dedicated-cluster error.

Testing

• go test ./... -count=1
• git diff --check

Caveat

Legacy dedicated grouped clusters reject individual database credential rotation (HTTP 400). The CLI propagates that error without silently rotating group credentials. Their group rotation can affect database tokens through a shared cluster key, which is why the guidance says "not necessarily".

Related: #1064 touches the same command with a server-type gate; this PR uses the database endpoint as the source of support instead.

Amp thread: https://ampcode.com/threads/T-01a0e943-2130-70ed-8b52-4fb27500934a