Fix terminal auto-activation migration to target User settings

[Copilot]

The legacy activation migration omitted its configuration target, causing workspace-open sessions to attempt a prohibited Workspace settings write. The synchronous getter also dropped the update promise and retried failed writes on subsequent reads.

• Migration

  • Run the migration once during awaited extension activation.
  • Write through the existing User/global-scoped setter:

    await setAutoActivationType(ACT_TYPE_OFF);

  • Handle write failures without retrying from configuration reads.

• Precedence

  • Skip migration when python-envs.terminal.autoActivationType is explicitly set.
  • Preserve the legacy false fallback behavior while keeping the getter side-effect free.

• Regression coverage

  • Assert the write targets User settings.
  • Verify failed migrations are not retried by repeated reads.

• Fixes python-envs.terminal.autoActivationType migration script incorrectly targets Workspace settings instead of User settings #1608