fix(observer): register the hooks again when the SAPI restarts the module

[ptondereau]

Description

Follow-up to #807: since a SAPI restart resets the engine's observer lists and exception hook, the fcall, error and exception observers now register their hooks again on every MINIT with the same instance, instead of going silent after the first restart. It also restores RefUnwindSafe on ClassMetadata, lost in #807 and the reason #810 plans 0.17.0, uses release/acquire ordering for the class entry pointer and documents that class entries are valid only until the module shuts down.

Checklist

• I have read the contribution guidelines and the code of conduct.
• I have added tests that prove my code works as expected.
• I have added documentation if applicable.
• I have added a migration guide if applicable.

[coveralls]

Coverage Report for CI Build 37858687091

Warning

No base build found for commit baa0a7c on master.
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Coverage: 74.488%

Details

• Patch coverage: 4 uncovered changes across 2 files (28 of 32 lines covered, 87.5%).

Uncovered Changes

File	Changed	Covered	%
src/zend/exception_observer.rs	15	12	80.0%
src/class.rs	9	8	88.89%
Total (4 files)	32	28	87.5%

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	14605
Covered Lines:	10879
Line Coverage:	74.49%
Coverage Strength:	53.62 hits per line

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💛 - Coveralls

[codspeed]

Merging this PR will not alter performance

✅ 45 untouched benchmarks

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_{Comparing fix/observer-module-restart (1a8fee0) with master (baa0a7c)}