fix: surface actual error message in ntfy test connection

[Siumauricio]

Summary

• The ntfy test connection catch block was returning a generic "Error testing the notification" message, hiding the actual error from the ntfy server (SSL, DNS, auth, etc.)
• Now includes the underlying error message in the tRPC response so users can actually diagnose the issue

Closes #4047

Greptile Summary

This PR fixes the testNtfyConnection tRPC mutation to surface the underlying error message (SSL, DNS, auth, etc.) instead of the generic \"Error testing the notification\" fallback, directly addressing issue #4047. The change is minimal, correct, and non-breaking.

• The fix is valid and safe to merge
• 10 other notification test handlers (testTelegramConnection, testGotifyConnection, testMattermostConnection, testLarkConnection, testPushoverConnection, and more) still swallow the real error behind the same generic message
• testSlackConnection already surfaces errors but uses a different format (no \"Error testing...\" prefix, \"Unknown error\" fallback), creating a minor inconsistency across handlers

Confidence Score: 4/5

Safe to merge — the change is a small, targeted improvement to error reporting with no functional risk.

The change is correct and non-breaking. The only concern is incomplete coverage (other handlers still use generic messages), but this does not block the fix from being merged.

No files require special attention; the single changed file (notification.ts) contains the complete, self-contained fix.

_{Reviews (1): Last reviewed commit: "fix: surface actual error message in ntf..." | Re-trigger Greptile}

Greptile also left 1 inline comment on this PR.

[greptile-apps]

Fix only applied to ntfy — other handlers still hide errors

This fix correctly surfaces the actual error message for testNtfyConnection, but the same problem exists in 10 other test connection handlers that still return the generic "Error testing the notification" message:

• testTelegramConnection (~line 218)
• testDiscordConnection (~line 282)
• testEmailConnection (~line 352)
• testResendConnection (~line 418)
• testGotifyConnection (~line 611)
• testMattermostConnection (~line 749)
• testCustomConnection (~line 804)
• testLarkConnection (~line 876)
• testTeamsConnection (~line 935)
• testPushoverConnection (~line 1009)

Additionally, testSlackConnection (~line 154) already surfaces errors but with a slightly different format — no prefix and "Unknown error" as the fallback — creating a minor inconsistency:

// testSlackConnection (existing)
message: `${error instanceof Error ? error.message : "Unknown error"}`

// testNtfyConnection (this PR)
message: error instanceof Error
  ? `Error testing the notification: ${error.message}`
  : "Error testing the notification"

Consider extending this fix to all test connection handlers and aligning on a consistent error message format.