Fix Context cycle collection and GC ownership

[EdmondDantes]

Contexts that retain themselves as a value or object key cannot be collected because Context does not expose its internal references to Zend GC. Coroutine and Scope also expose the context's contents instead of their owned reference to the Context object.

Add Context's GC handler, enumerate the actual Context ownership edges from Coroutine and Scope, and release the context tables in free_obj so they remain valid during GC's traversal after destructors.

The regression test checks automatic collection of standalone value/key cycles and collection of contexts owned by completed coroutines and disposed scopes.

Validation: 94 Context/Scope/GC PHPT passed with four workers on PHP 8.6.0-dev, ZTS debug, TrueAsync ABI 0.26.0. git diff --check passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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