refactor: unify query building logic for merge suggestions [CM-765]

[skwowet]

Changes

This PR builds on the previous refactoring effort to further improve the merge suggestion query logic. The focus is on unifying the implementation for members and organizations to make it more robust and maintainable.

Before:

• Member and organization query building logic was handled in separate, lengthy blocks of code.
• Some exact match queries for members were still not chunked, causing maxClauseCount errors to persist in edge cases.
• Identity processing and cleaning logic was mixed with query construction.

After:

• A generic clauseBuilders pattern has been introduced to create a unified and DRY implementation for both member and organization queries.
• All query types are now consistently chunked, fully resolving the OpenSearch maxClauseCount issue for both members and organizations.
• Helper functions for identity processing (isNumeric, cleanIdentityValue, etc.) have been extracted to improve clarity.
• Shared constants like EMAIL_AS_USERNAME_PLATFORMS have been moved to a central enums.ts file.

This refactor ensures that the suggestion generation is both efficient and easier to maintain going forward, without affecting the quality of the results.