Align UserSettings L2 coverage with 6-digit ContentPin validation

[Copilot]

Compared test-github-mode to develop, the only behavioral source change is SetContentPin tightening validation from 4 digits to 6 digits. L2 tests still treated a 4-digit PIN as success, so coverage no longer matched implementation behavior.

• Coverage gap addressed

  • Updated the existing COM-RPC success-path L2 assertion to use a valid 6-digit PIN.
  • Updated the corresponding GetContentPin expectation to assert the stored 6-digit value.

• New negative-path coverage

  • Added an invalid-input assertion for 4-digit PINs to verify Core::ERROR_INVALID_PARAMETER.
  • Added a follow-up read to ensure failed invalid input does not overwrite the previously stored valid PIN.

• Illustrative delta

  status = m_usersettingsplugin->SetContentPin("123456");
  EXPECT_EQ(status, Core::ERROR_NONE);
  
  status = m_usersettingsplugin->SetContentPin("1234");
  EXPECT_EQ(status, Core::ERROR_INVALID_PARAMETER);
  
  status = m_usersettingsplugin->GetContentPin(getStringValue);
  EXPECT_EQ(getStringValue, "123456");

L2-Guardian-Issue: #