THRIFT-6370: PHP: Give each TCurlClient its own curl handle

[sveneld]

TCurlClient instances currently share a static curl handle. If client A sets a 100 ms timeout, a new client B with no timeout inherits it: a response that arrives after 350 ms fails after about 100 ms. The connection timeout is shared in the same way.

Give each client its own reusable curl handle. Closing a client or handling a failed transfer releases only that client's handle. Remove the global shutdown callback so PHP can release the handle with its owning object.

JIRA: THRIFT-6370

Task: PHP: Give each TCurlClient its own curl handle

Acceptance criteria:

• Timeout and connection-timeout settings do not carry over to another client.
• Repeated requests reuse the owning client's handle.
• Closing a client or failing its request leaves other clients' handles usable.
• A subsequent request recreates a released handle.

Compatibility: closeCurlHandle() becomes an instance method. Replace TCurlClient::closeCurlHandle() with $client->closeCurlHandle(), or use $client->close() to release its handle and clear its buffers. The PHP README documents this change. Connections are reused within a client instance rather than shared across instances.

Validation on PHP 8.2.30 with PHPUnit 10.5.65:

• The new timeout-isolation regression test failed before the implementation change, showing B using A's handle and both timeout settings.
• All 35 TCurlClientTest cases and 4 TCurlClientRedirectTest cases pass after the change.
• PHPCS passes for the changed PHP files; git diff --check is clean.
• make style was attempted but is unavailable in the unconfigured worktree (No rule to make target 'style').

Implementation and regression tests were AI-assisted.

Generated-by: Codex (GPT-6)