Align Content PIN validation with 6-digit requirement and close L2 coverage gaps

[Copilot]

Compared test-github-mode against develop and found a production behavior change (SetContentPin now expects 6 digits) without matching L2 updates. This PR brings L2 coverage in sync with that change and validates boundary behavior through COM-RPC paths.

• Production behavior alignment

  • Updated UserSettingsImplementation::SetContentPin to enforce a 6-digit numeric PIN format.

• L2 test updates for existing happy paths

  • Updated existing L2 scenarios that previously used 1234 to use 123456 where success is expected.

• New L2 coverage for input validation semantics

  • Added setContentPinValidatesInputFormatComRpc to verify:
    • valid 6-digit PIN is accepted
    • invalid PINs (1234, 12345, 1234567, 12ab56) are rejected with Core::ERROR_INVALID_PARAMETER
    • invalid writes do not overwrite the last valid stored PIN
    • empty string remains accepted/persisted as supported behavior

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(contentPin);
EXPECT_EQ(contentPin, "123456");

L2-Guardian-Issue: #