fix(errors): the pool-exhausted StateError is shown raw, a reset connection reads as "did not accept the connection", and TLS errors have no hint #1316

Problem

lib/core/database/database_error_mapper.dart and the pools:

1. Raw Bad state:. The pools rethrow StateError('... connection pool exhausted: 32 slots in use.') (on StateError { rethrow; }) and it reaches the UI as Bad state: PostgreSQL connection pool exhausted....
2. Reset vs refused. Any SocketException becomes HostUnreachableException (did not accept the connection), including Connection reset by peer and Broken pipe on a session that was working: the server closed it, it did not refuse it.
3. TLS. HandshakeException, CERTIFICATE_VERIFY_FAILED, unknown ca, hostname mismatch are unmapped and show the raw text; the fix (SSL mode, root certificate) is not suggested.

Scope

• Map pool exhaustion to a typed error with a hint (close unused tabs or connections).
• A connection lost error for reset / broken pipe / EOF on an established session, with a reconnect hint.
• A TLS error class with a hint pointing at the SSL mode and root certificate fields.
• Mapper tests for each.

Acceptance

• No user-facing message starts with Bad state: for pool exhaustion.
• Connection reset by peer is not reported as refused.
• A certificate verification failure mentions the SSL mode / root certificate.