feat(zend): add output handlers

[ptondereau]

Description

Add zend::start_output_handler, a safe wrapper around php_output_handler_create_internal (same API from 8.1 to 8.5) where the closure borrows the buffered bytes without a copy and returns a Cow, and a borrowed start of the input goes back to the engine without a copy. The engine owns every output buffer it gets, so neither a header callback that runs the handler again nor a fatal error swallowed in the closure can leave it with a freed pointer.

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.

[codspeed]

Merging this PR will not alter performance

⚠️ 51 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• output_passthroughs[1]
• output_passthroughs[10]
• output_passthroughs[100000]
• output_transforms[1]
• output_transforms[10]
• output_transforms[100000]
• callback_calls[100000]
• exception_throws[100000]
• array_str_ref_keys[100000]
• property_dumps[100000]
• …and 41 more

✅ 45 untouched benchmarks
🆕 6 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕 ⚠️	output_passthroughs[1]	N/A	23.6 ms	N/A
🆕 ⚠️	output_passthroughs[10]	N/A	23.6 ms	N/A
🆕 ⚠️	output_passthroughs[100000]	N/A	155.9 ms	N/A
🆕 ⚠️	output_transforms[1]	N/A	23.6 ms	N/A
🆕 ⚠️	output_transforms[10]	N/A	23.6 ms	N/A
🆕 ⚠️	output_transforms[100000]	N/A	217.6 ms	N/A

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_{Comparing feat/output-handler (73154c4) with master (ebdd099)}

[coveralls]

Coverage Report for CI Build 38028300055

Coverage increased (+0.4%) to 74.87%

Details

• Coverage increased (+0.4%) from the base build.
• Patch coverage: 13 uncovered changes across 1 file (228 of 241 lines covered, 94.61%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
src/zend/output.rs	241	228	94.61%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
crates/introspection/src/stub.rs	1	88.46%

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Coverage Stats

Relevant Lines:	14843
Covered Lines:	11113
Line Coverage:	74.87%
Coverage Strength:	53.31 hits per line

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💛 - Coveralls