Fix VIES integrity check: tolerate country-code prefix on echoed VAT number (uptake fix)

[dcenic]

What & why

Follow-up fix to #11435 (merged). The VIES response-integrity check added there compared the echoed vatNumber against VAT Registration Log.GetVATRegNo(), which strips the leading country-code prefix. A VIES response can echo the number with or without that prefix (member states and the test mock differ), so a response echoing DE813261484 was wrongly rejected as "did not match the requested identifiers" against the stripped request value 813261484.

Because ValidateResponseIntegrity runs inside the VAT Registration No. OnValidate, this didn't just fail a test — it hard-blocked saving the VAT number.

Fix

Normalize both sides to the prefix-stripped form before comparing (strip the response's own echoed country-code prefix from its vatNumber). A cosmetic prefix difference is no longer a mismatch, while a genuinely different number or country is still rejected (security intent preserved).

Impact

Fixes the MOCKSERVICETESTS-Internal ERM VAT VIES Lookup WebService tests (codeunit 134192) failing in BCApps uptake in the NAV repo (10 tests). That bucket needs a live mock service and does not run in the BCApps PR gate, which is why the regression wasn't caught here before merge.

Validation

• AL diagnostics clean.
• Traced all 10 failing mocks (DE/DE813261484) -> now pass; reject/mismatch unit tests still reject.
• Full end-to-end runs in the NAV uptake gate (mock service required). Verified this locally, ran the affected test codeunit 134192 with MockService running locally. All pass.
• Codeunit 134192 runs in NAV but not in BCApps because it needs MockService. It needs it because it is mocking a SOAP request and cannot use HttpRequestHandler pattern for mocking.

Fixes AB#650565