feat(routes-b): implement DELETE /api/routes-b/email-suppressions/[id]

[QueenColly]

Summary
This PR implements the DELETE /api/routes-b/email-suppressions/[id] endpoint under the routes-b convention. It handles secure deletion of email suppressions with appropriate request validation, ownership checks, and structured error envelopes.

Changes Proposed

Created route handler at app/api/routes-b/email-suppressions/[id]/route.ts.

Implemented the DELETE method with structured response and error envelopes matching existing route conventions.

Added corresponding unit tests covering success states and validation error handling.

Acceptance Criteria Checklist

[x] Handler implemented for the DELETE method at the documented path.

[x] Validation and error envelopes match existing routes conventions.

[x] Unit tests pass in CI.

Closes #1133

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