Add SalesOrdersEndpoint::delete()

[loevgaard]

What

Adds a delete operation to the sales-orders endpoint:

$client->salesOrders()->delete(37707008); // DELETE /sales_orders/{id}

Mirrors the existing WebhooksEndpoint::delete() exactly — delete(int $id): void, routed through Client::delete() (which tolerates an empty response body). No interface changes needed (endpoints expose plain methods on the leaf).

Tests

SalesOrdersEndpointTest::it_deletes_a_sales_order_by_id — asserts the DELETE method and /sales_orders/5 URL via the scripted HTTP fake (same shape as the webhook delete test). No live test added — deleting against the real API is destructive.

Verification

• PHPUnit: 107 tests / 250 assertions (3 live-API skipped)
• PHPStan max: clean
• ECS: clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.08%. Comparing base (0bf7a00) to head (f34602f).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x      #10   +/-   ##
=========================================
  Coverage     98.07%   98.08%           
- Complexity      186      187    +1     
=========================================
  Files            33       33           
  Lines           520      522    +2     
=========================================
+ Hits            510      512    +2     
  Misses           10       10           

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