fix: explain missing connector key at startup

[Chris0Jeky]

Summary

• classify the existing missing Connectors:EncryptionKey validation failure in packaged desktop fatal output
• provide a stable actionable message naming Connectors__EncryptionKey without printing exception content or secrets
• add a regression proving the cause-specific output and secret-free boundary
• leave headless data-root/restart policy, config docs, and secret generation unchanged

Verification

• baseline DesktopRuntimeTests: 29 passed, 0 skipped
• focused final DesktopRuntimeTests: 30 passed, 0 skipped
• final FirstRun namespace: 137 passed, 0 skipped
• git diff --check: passed
• two coordinator review passes: no blocking findings

Automatic Codex review is unavailable because the account review quota is exhausted. Hosted checks remain pending.

Refs #2181

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.