fix!: keep Rust panics on the Rust side of the FFI boundary

[ptondereau]

Description

A Rust panic reached from PHP is now caught at every entry point the crate writes (generated handlers, constructors, closures, object handlers) and thrown as a PHP Error with the message Rust panic: <message>, instead of aborting the PHP process; PhpException::throw is infallible, try_catch reports CatchError::Panic, and a failing module startup returns the real Zend FAILURE so PHP refuses to start the module. Hooks and raw extern "C" callbacks the user hands to the engine still abort, which the guide and the v0.16 migration guide now document.

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 35506326831

Coverage decreased (-0.1%) to 69.349%

Details

• Coverage decreased (-0.1%) from the base build.
• Patch coverage: 102 uncovered changes across 9 files (71 of 173 lines covered, 41.04%).
• 14 coverage regressions across 6 files.

Uncovered Changes

File	Changed	Covered	%
src/zend/handlers.rs	48	0	0.0%
src/closure.rs	17	0	0.0%
src/zend/try_catch.rs	40	27	67.5%
src/exception.rs	8	0	0.0%
src/internal/mod.rs	33	27	81.82%
src/builders/class.rs	4	0	0.0%
src/zend/ce.rs	3	0	0.0%
src/convert.rs	2	0	0.0%
src/php_eval.rs	1	0	0.0%
Total (11 files)	173	71	41.04%

Coverage Regressions

14 previously-covered lines in 6 files lost coverage.

File	Lines Losing Coverage	Coverage
src/exception.rs	5	83.55%
src/builders/class.rs	2	77.44%
src/closure.rs	2	20.19%
src/php_eval.rs	2	97.04%
src/zend/handlers.rs	2	0.0%
src/zend/try_catch.rs	1	89.51%

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Coverage Stats

Relevant Lines:	13771
Covered Lines:	9550
Line Coverage:	69.35%
Coverage Strength:	43.49 hits per line

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💛 - Coveralls

[codspeed]

Merging this PR will not alter performance

✅ 30 untouched benchmarks

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_{Comparing fix/ffi-panic-policy (7543b95) with master (645078e)}