Optimized the type of Content-Type to string from int for Hyperf\Tracer\Adapter\Reporter\HttpClientFactory. - #7791

What problem does this PR solve?

Guzzle 7.11 deprecates non-string request header values, and Guzzle 8 requires header values to be strings or non-empty arrays. The Zipkin HTTP reporter currently assigns the integer returned by strlen($payload) to Content-Length.

In coroutine workloads, this deprecation may be captured by another active error handler and surface as an unrelated stream read failure.

What is changed and how does it work?

Cast the payload length to a string before passing the headers to Guzzle:

'Content-Length' => (string) strlen($payload),

Tests

Added HttpClientFactoryTest::testContentLengthHeaderIsString to verify the reporter sends Content-Length as a string.

• Before the fix: Failed asserting that 7 is identical to '7'.
• After the fix: tracer test suite passes with 5 tests and 5 assertions on PHP 8.2.28 with Swoole.
• PHPStan reports no errors.
• PHP CS Fixer reports no required changes.