fix(case-helpers): preserve custom_data keys at any nesting depth

[TabishRiazBajwa]

Problem

convertToSnakeCase in src/internal/api/case-helpers.ts has a carve-out that preserves customData contents when it appears as a top-level key on the request body. This works correctly for paddle.subscriptions.create(), paddle.transactions.create(), etc.

However, when creating a simulation via paddle.simulations.create(), customData is nested inside payload.data — not at the top level. The existing guard never fires, so all camelCase keys inside customData get silently converted to snake_case before being sent to the Paddle API.

Result: simulation webhooks deliver { tenant_id: "abc" } instead of { tenantId: "abc" }, breaking any webhook handler that reads theoriginal keys — and producing a silent data corruption that's hard to debug.

Fix

Move the customData preservation logic into decamelizeKeys so it applies at any depth of the object tree, not just the top level. When the recursive walk encounters an object key named customData, it copies the value as-is (snake-cased key, original value) instead of recursing into it.

Testing

• Added a unit test covering convertToSnakeCase with customData nested inside a simulation payload.data object.
• Verified existing top-level customData tests still pass.

Fixes #220