fix(tests): reset middleware order header per request (closes #533)

[s2x]

Description

Closes #533

Changes

• Reset the test middleware execution-order accumulator on the first middleware invocation so MiddlewareTest::testHeaders no longer sees a stale X-Test-Middleware-request-order value on subsequent keep-alive requests under macOS / Workerman
• Added a regression test (testMiddlewareRequestHeadersDoNotLeakAcrossRequests) that performs two consecutive requests through the same HTTP client and asserts identical middleware order on both responses

Root cause

The test middleware (TestMiddleware) accumulated the X-Test-Middleware-request-order header across keep-alive requests because the request header state was never reset at the start of each request. On macOS / Workerman the second request received the accumulated value, causing testHeaders to see the chain 3× instead of 1×.

Changelog

• Fixed: reset middleware order header per request ([Bug] MiddlewareTest::testHeaders fails on macOS — middleware executes 3× instead of 1× #533)

Code Review

• Passed subagent code review
• All review comments addressed