Fix AOT LLVM discovery in Docker; add str_contains AOT PHPT

[PurHur]

Summary

• LLVM 9 resolution: AOT/LLVM PHPUnit code only checked repo/.llvm, so tests in the php-compiler:22.04-dev image (toolchain at /opt/llvm9) were skipped silently (45 skipped, 0 assertions). Added test/LlvmToolchain.php mirroring script/php-env.sh (repo .llvm → PHP_COMPILER_LLVM_PATH → /opt/llvm9) and wired it through BaseTest, all test/aot/*, ServeAotTest, ExamplesCompileTest, and JITTest setup.
• Bootstrap: src/llvm-env.php now uses the same resolution order so in-process PHPLLVM loads in Docker.
• New coverage: test/fixtures/aot/cases/str_contains.phpt for str_contains() (web routing / header checks).

Test output (Docker, Runforge harness)

Harness bind-mount is empty; used ./script/docker-ci-local.sh pattern (tar pipe):

$ tar -cf - --exclude='.git' --exclude='.llvm' . | docker run --rm -i -w /compiler php-compiler:22.04-dev \
    bash -c 'tar -xf - && script/apply-patches.sh 2>/dev/null; php vendor/bin/phpunit test/aot/'

OK (46 tests, 86 assertions)

Before this change, the same command reported Skipped: 45 with OK, but incomplete.

LLVM 9 reported at /opt/llvm9 by script/ci-local.sh when PHP_COMPILER_LLVM_PATH is set in the image.

Test plan

• php vendor/bin/phpunit test/aot/ in php-compiler:22.04-dev (46 tests, 86 assertions)
• Full ./script/ci-local.sh in Docker (recommended before merge)

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