chore: fix gRPC fork deadlock in Ubuntu unit tests

[bshaffer]

Description

This PR fixes the issue causing the Unit tests in Ubuntu (and Package tests) on PR #9514 to hang and time out after 6 hours.

Root Cause

1. Auth/tests/Cache/RaceConditionTest.php uses pcntl_fork() to test concurrency across 4 child processes.
2. In the monorepo CI, the PHP grpc extension is installed for Ubuntu runners.
3. When the grpc C-core extension is loaded in a PHP process, background threads and mutexes are initialized. When pcntl_fork() is called, POSIX fork() only clones the calling thread, leaving unreleased locks and absent background worker threads in the child process.
4. When the child process calls exit(0), PHP runs module shutdown, which deadlocks indefinitely on a futex (futex_do_wait) waiting on mutexes/threads that do not exist in the child.
5. The parent process is stuck in pcntl_waitpid(), and PHPUnit is stuck waiting for the test process to finish, causing the 6-hour timeout in CI.
6. This also hung run-package-tests.sh because Auth is included in the package test directory loop and run under xargs.
7. This passed on Windows because pcntl_fork is unsupported on Windows (test was skipped), and passed on PHP 8.1 Unit Test (no gRPC) because grpc was not installed.

Solution

1. Add GRPC_ENABLE_FORK_SUPPORT=1 to unit-tests.yaml, run-package-tests.sh, phpunit.xml.dist, and Auth/phpunit.xml.dist. This enables gRPC's pthread_atfork handlers to properly reset C-core state post-fork so child processes exit cleanly.
2. Add a defensive skip check in Auth/tests/Cache/RaceConditionTest.php so that if a developer runs PHPUnit locally with grpc loaded and without GRPC_ENABLE_FORK_SUPPORT=1, the test skips with an informative message instead of hanging.