feat: add account_type field to ProductSearch for Rev 6 account_type filtering | MC-16677

[SandhyaBhatia]

Summary of Changes

Adds an account_type field to ProductSearch so MDX Product's list endpoint (GET /users/{user_id}/products?type=&account_type=) can bind and pass through the account_type query parameter, per the Rev 6 MDX Product spec.

Fixes MC-16677 (https://mxcom.atlassian.net/browse/MC-16677)

Public API Additions/Changes

• ProductSearch gains getAccount_type() / setAccount_type(String).
• Named account_type (not accountType) intentionally — this controller's query-param binding (ProductController#listProducts(ProductSearch)) does Spring's default property-name matching with no snake_case/camelCase conversion. Confirmed by existing precedent: TransactionSearchRequest.start_date uses the same non-standard naming for the same reason (see its class comment). @SuppressWarnings({"checkstyle:MemberName", ...}) was already present on ProductSearch for this purpose.

Downstream Consumer Impact

• Purely additive — no existing fields/behavior changed. Accessors can now read productSearch.getAccount_type() to filter product lists by account type; accessors that don't use it are unaffected.
• path-accessor-central-pacific's ProductAccessor.list() will consume this field (tracked in MC-16678, separate PR).

How Has This Been Tested?

• ./gradlew :mdx-models:test — all 575 existing tests pass.
• ./gradlew :mdx-models:compileJava :mdx-models:checkstyleMain — clean (only pre-existing unrelated deprecation warnings).
• No new dedicated unit test added — this module has no existing per-field tests for simple search/POJO models (ProductSearch, TransactionSearchRequest, etc. have none), so this follows existing convention.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works