L2 guardian: close UserSettings content PIN coverage gap for 6-digit validation

[Copilot]

Compared test-github-mode against develop, the only production delta was SetContentPin moving from 4-digit to 6-digit regex validation. This PR updates L2 coverage to match that contract so changed behavior is exercised on both accept and reject paths.

• Scope of source change analyzed

  • plugin/UserSettingsImplementation.cpp: SetContentPin validation changed from ^\d{4}$ to ^\d{6}$.

• L2 test alignment updates

  • Updated existing L2 success-path PIN inputs from 1234 → 123456 in:
    • migration-state flow setup
    • JSON-RPC onContentPinChanged flow
    • COM-RPC set/get flow

• New negative-path validation coverage

  • Expanded setContentPinInvalidLengthErrorCase to assert rejection of:
    • non-6-digit numeric inputs
    • malformed 6-char inputs (non-numeric / whitespace / symbols)
    • excessive-length numeric input
  • Added explicit positive checks in the same coverage area for:
    • valid 6-digit PIN acceptance
    • empty-string acceptance (existing plugin behavior)

paramsContentPin["contentPin"] = "123456";
EXPECT_EQ(InvokeServiceMethod("org.rdk.UserSettings", "setContentPin", paramsContentPin, result_json), Core::ERROR_NONE);

paramsContentPin["contentPin"] = "12345";
EXPECT_EQ(InvokeServiceMethod("org.rdk.UserSettings", "setContentPin", paramsContentPin, result_json), Core::ERROR_INVALID_PARAMETER);

L2-Guardian-Issue: #