fix(extensions): edit-connection UI parity (password keep, Test, sidebar Remove)

[ZhuchkaTriplesix]

Problem

Follow-up to #510 / PR #517. Extension Edit connection… matches the menu and footer of built-in forms, but several UI/behavior gaps break parity — especially around secrets.

Gaps

1. Password “keep existing” (blocker)

Built-in edit forms allow an empty password and show Leave blank to keep existing; mergeSecretsForConnectionUpdate restores the stored secret after Save.

Extension SDUI password fields:

• Use the schema placeholder only (no keep-existing hint)
• Honor field.required — empty password fails validation, so Save can be blocked without retyping the secret

Storage merge after a successful pop is fine; the form never reaches it when validation fails.

2. Test Connection ignores secret merge

In edit mode, Test Connection builds a row from form values without merging the previous password. Save path merges; Test can fail with a blank password even when Save would succeed.

3. Dual Remove on extension tile

Built-in tiles: Remove only via context menu.
Extension tile: context menu Remove and inline × — two delete affordances.

Proposed fix

1. SDUI edit mode (SduiFormBuilder or edit wrapper in extension_connection_form.dart):
   • Password fields: empty is valid while editing
   • Placeholder / hint: Leave blank to keep existing (same copy as built-ins)
2. Test Connection (edit): call mergeSecretsForConnectionUpdate (or inject previous password) before ExtensionDriverSession.testConnection
3. Sidebar: remove the inline × on the extension tile (prefer one dialect: menu-only, matching built-ins)

Acceptance

• Editing an extension connection with a required password field: leave password empty → Save succeeds and keeps the stored secret
• Password field shows keep-existing hint in edit mode
• Test Connection with blank password in edit mode uses the stored secret (same outcome as Save would)
• Extension connection tile has a single Remove affordance (context menu only, unless product decides otherwise for all drivers)
• Widget/unit tests cover blank-password validation + secret merge on Test where practical

Files (starting points)

• lib/features/connections/extension_connection_form.dart
• lib/core/sdui/sdui_form_builder.dart
• lib/features/connections/connections_panel_extension.dart
• lib/features/connections/connection_creation_flow.dart (mergeSecretsForConnectionUpdate)

Out of scope (optional later)

• Migrate connection form shells to QueryaDialogCard
• Map SDUI text/password controls to shadcn TextField for full chrome parity
• Secrets stored only under custom driverOptions keys (not top-level ConnectionRow.password)

Related

• Parent: feat(connections): edit existing connection (host, auth, SSL, …) #510
• Review notes: extension edit UI dialect vs built-ins

[ZhuchkaTriplesix]

Shipped in #519