Unpack AggregateError inner errors in formatError for better diagnostics

[juergen-albertsen-exasol]

Summary

When connecting to an unreachable Exasol host, the output panel and error messages would show AggregateError with no useful detail. This is because AggregateError (thrown when all WebSocket connection attempts fail) wraps multiple inner errors in an .errors array, which the previous formatError implementation ignored.

Changes

• formatError now detects AggregateError and recursively unpacks its inner errors into a readable string, e.g. All connection attempts failed: connect ECONNREFUSED 10.0.0.1:8563; connect ETIMEDOUT 10.0.0.2:8563
• extension.ts and connectionPanel.ts replaced bare String(error) calls with formatError(error), so all error display paths benefit from the same logic
• Two unit tests added to cover the new behaviour

[mkcorneli]

• Correctness: The AggregateError check is placed before the instanceof Error check, which is correct since
  AggregateError extends Error — otherwise it'd never trigger. The recursive formatError(e) call on inner errors
  means nested AggregateErrors and non-Error inner values are handled properly.
• Import paths: extension.ts imports directly from connectionTypes, connectionPanel.ts imports via the
  connectionManager re-export — both are valid, though slightly inconsistent. Minor style nit, not a blocker.
• Tests: Two good unit tests covering the happy path (message + inner errors) and edge case (no message). Test
  file placement in the existing tlsValidation.test.ts suite makes sense since formatError tests already live
  there.
• No regressions: The String(error) → formatError(error) replacements are strictly better — formatError
  already handles all the cases String() would, plus the driver's {text, sqlCode} objects.

[mikhail-zhadanov]

This PR didn't add anything to changelog so no new release was generated.

[juergen-albertsen-exasol]

This PR didn't add anything to changelog so no new release was generated.

Not sure what you're trying to say. What do we need to do?

CC @mkcorneli

[mikhail-zhadanov]

@juergen-albertsen-exasol , CI is automatically deploying new release to marketplace if

1. version number was updated in https://github.com/exasol-labs/exasol-vscode/blob/main/package.json
2. changes recorded in https://github.com/exasol-labs/exasol-vscode/blob/main/CHANGELOG.md

If not done - changes stay only in github code.

[juergen-albertsen-exasol]

I created a PR to bump the version: #40