Fix stale integration test fixtures

[lucasmiranda-stark]

Context

Preflight on master surfaced three failing integration tests in core_clojure.rest-test: rest-post, rest-post-single, rest-post-sub-resource. All three are test-side fixture issues — the SDK source is correct.

What changed

• test/core_clojure/rest_test.clj: rest-post and rest-post-single now POST /webhook with delete-id cleanup
• test/core_clojure/rest_test.clj: rest-post-sub-resource installment totalAmount values raised to meet the API's >=500-per-installment minimum; purchase amount aligned with the chosen installmentCount
• CHANGELOG.md: [Unreleased] Fixed entry

Test plan

• lein test on master: 0 failures, 3 errors
• lein test on this branch: 0 failures, 0 errors

Risk & Rollback

• Risk: test-only changes
• Rollback: revert this PR

Branch / Base

• Branch: fix/stale-integration-test-fixtures
• Base SHA: be170ec49f22e4adcc999fba7a7c74ebfee20f3d