Replace string-matched error classification with typed errors

[andreagrandi]

Summary

JSON error classification previously relied on matching substrings in error messages, making --error-format json output brittle when wording changed (e.g. a 401 body of Unauthenticated vs Unauthorized).

This introduces a new internal/mberr package with typed errors and rewrites classification to inspect error types via errors.As instead of message text.

Changes

• New internal/mberr package with typed errors:
  • ConfigError — missing/invalid environment configuration
  • APIError — non-2xx responses, with IsAuth() for 401/403
  • TimeoutError / CanceledError — context deadline/cancellation
  • RequestError — wraps a failure for a specific ResourceKind (database, table, field, dashboard, card, dashboard parameter)
  • ResolutionError — name→ID resolution failures
  • ParameterizedQueryError — rejected parameterized queries
• classifyError now matches error types via errors.As; resource-specific suggestions derive from ResourceKind rather than substring matches.
• Error producers updated to return typed errors: config.go, client.go, cards.go, dashboards.go, query.go, card.go.

Testing

• make build, make vet, gofmt — clean
• tests/error_format_test.go rewritten to construct typed errors directly; 19 subtests cover config, auth, API (incl. resource-specific 404s), resolution, timeout/cancellation, and errors.As unwrapping of wrapped errors
• Smoke-tested all four acceptance-criteria categories against a live API: CONFIG_ERROR, AUTH_ERROR, API_ERROR (404 with resource-specific suggestion), RESOLUTION_ERROR

Acceptance criteria

• Structured JSON errors do not depend on fragile substring matching
• Tests cover config, auth, API, and resolution error classification
• Existing --error-format json behavior remains compatible

Closes #12