feat(http): allow injecting Guzzle handler; assert customer id on the wire [BIL-1509]

[fjalil7]

Summary

• HttpClient constructor now accepts an optional ?HandlerStack $handler for testing
• Wire-level tests added (tests/CustomersTest.php) using MockHandler + Middleware::history to capture the real wire body
• PHP already sent id correctly — this adds the regression test and the handler seam

Test plan

• vendor/bin/phpunit — 43 tests pass
• Wire-level test captures the Guzzle request body after convertKeys (camelCase) and asserts billingEmail and id are present

Closes BIL-1509

[linear-code]

BIL-1509