php-in-php: gate Context string helpers on thin/init-phase (#20571)

[PurHur]

Summary

• Gate Context::ensureFullStandaloneBodies NestedJIT string helpers on isThinStandaloneAotMain() + StreamIoRuntime::isStandaloneInitPhase() instead of the broad shouldDeferHeavyStreamIoEmitters bag (peer php-in-php: drop shouldDeferHeavyStreamIoEmitters from StreamRead/Lifecycle/Bucket/Filter — thin via isThinStandaloneAotMain (#1492) #20553).
• Add ContextStandaloneBodiesRuntimeShrinkTest so Context never re-imports the StreamIo defer bag.

C / defer shrink: removed the last StreamIoRuntime::shouldDeferHeavyStreamIoEmitters call site from lib/JIT/Context.php (string-helper block). Observable init-phase skip unchanged.

Closes #20571

Test plan

• vendor/bin/phpunit test/unit/ContextStandaloneBodiesRuntimeShrinkTest.php — OK (2 tests)
• vendor/bin/phpunit test/unit/FileGetContentsRuntimeShrinkTest.php — OK
• ExamplesHelloWorldAotRegressionTest Context/StreamIo assertion filters — OK
• php script/bootstrap-inventory.php --check — OK 5679/5679
• make bootstrap-selfhost-vm-driver-execute-probe — OK

Verification transcript

OK (2 tests, 6 assertions) ContextStandaloneBodiesRuntimeShrinkTest
OK (4 tests, 16 assertions) FileGetContentsRuntimeShrinkTest
OK (2 tests, 18 assertions) ExamplesHelloWorldAotRegressionTest filters
OK 5679/5679
bootstrap-selfhost-vm-driver-execute-probe: OK

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