Add missing L2 coverage for Content PIN 6-digit validation change

[Copilot]

This updates L2 coverage for the test-github-mode delta vs develop, where SetContentPin changed validation from 4 digits to 6 digits. Existing L2 success-path tests still encoded 4-digit assumptions, leaving a coverage gap around the new contract.

L2-Guardian-Issue: #

• Scope aligned to source delta

  • Focused on the only production change: UserSettingsImplementation::SetContentPin regex update (^\d{4}$ → ^\d{6}$).

• L2 test expectation realignment

  • Updated existing success-path L2 flows to use 6-digit PIN values ("123456") instead of 4-digit values.
  • Applied to JSON-RPC/event-path and COM-RPC set/get-path assertions that previously expected "1234".

• New focused contract tests

  • Added setContentPinWithFourDigitsReturnsInvalidParameter to lock the rejection path for legacy 4-digit input.
  • Added setContentPinWithSixDigitsReturnsSuccessAndPersists to lock valid acceptance + persisted readback behavior.

paramsContentPin["contentPin"] = "1234";
status = InvokeServiceMethod("org.rdk.UserSettings", "setContentPin", paramsContentPin, result_json);
EXPECT_EQ(status, Core::ERROR_INVALID_PARAMETER);

paramsContentPin["contentPin"] = "654321";
status = InvokeServiceMethod("org.rdk.UserSettings", "setContentPin", paramsContentPin, result_json);
EXPECT_EQ(status, Core::ERROR_NONE);