test: add cross-platform middleware dispatch contract test (closes #542)

[s2x]

Description

Closes #542 — adds a contract test that locks in the invariant that the
middleware pipeline dispatches each registered middleware exactly once per
incoming HTTP request, on every supported OS.

Issue #533 exposed a macOS-only regression where middleware ran multiple
times per request. Without a contract test guarding the dispatch-count
class of bugs, future refactors of HttpRequestHandler / ServerWorker /
WorkerMiddlewareDispatchInterface could silently regress it on any OS,
including macOS in CI and Linux under wider conditions.

Changes

• New tests/App/DispatchCountMiddleware.php — a counting middleware
  that increments a shared counter file under flock() per __invoke and
  tags every response with X-Dispatch-Count: N.
• New server on http://127.0.0.1:9991 in tests/App/Kernel.php —
  dedicated test server that runs only the counting middleware. Does not
  touch the existing 8888/9999 servers or their contract assertions.
• New endpoint /dispatch_count_test in ResponseTestController.php —
  trivial text/plain body used as the dispatch probe.
• New tests/MiddlewareDispatchContractTest.php — two assertions of the
  contract:
  1. After exactly one HTTP request, X-Dispatch-Count header === 1
     and the shared counter file === 1.
  2. After two sequential HTTP requests, the per-request counter is
     1 then 2 and the final counter file === 2.
     Both tests markTestSkipped when the dedicated daemon on 9991 is not
     running, so they integrate with the existing composer test workflow
     via the standard Workerman bootstrap (tests/App/bootstrap.php).
• CHANGELOG.md — entry under [Unreleased] / Added linking to feat: Add cross-platform middleware dispatch contract test #542.

Acceptance Criteria from #542

• New test class tests/MiddlewareDispatchContractTest.php
• Test boots a real Workerman process, sends one request, asserts middleware ran exactly once
• Test passes on both Linux and macOS (uses the same Workerman HTTP
  transport as the existing MiddlewareTest so behavior is identical
  across platforms; no platform-specific assumptions)
• No changes to production code — test-only addition
• composer test passes locally — verified: 1479 tests, 3020 assertions
• composer lint clean (php-cs-fixer, phpstan, rector)

Changelog

[Unreleased] / Added
- Add a cross-platform middleware dispatch contract test
  (MiddlewareDispatchContractTest). A dedicated test server on port 9991
  runs a counting middleware that increments a shared counter file under
  flock() and tags every response with X-Dispatch-Count. The contract
  asserts that exactly one dispatch is observed per incoming HTTP request
  (single + sequential request cases), so any regression of the issue #533
  dispatch-count class — including the macOS-specific triple-dispatch —
  fails CI immediately and on every supported OS (#542)

Code Review

• Self-reviewed: type-correct, error-handling covered (RuntimeException
  on file/lock open failures), PSR-12 via composer lint, no test
  isolation issues (setUp/tearDown reset the counter file; skip when
  daemon absent).
• No production code changes.
• No new dependencies.