fix(observer)!: merge observer ErrorType into flags::ErrorType

[ptondereau]

Description

The observer had its own ErrorType next to flags::ErrorType, so php_error and the prelude disagreed, and its ALL did not match PHP's E_ALL before 8.4. The observer now uses flags::ErrorType, which gains All, Core and Fatal from the PHP headers and is exported from the prelude without a feature gate.

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 36552728599

Coverage increased (+0.007%) to 70.575%

Details

• Coverage increased (+0.007%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (17 of 18 lines covered, 94.44%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/zend/error_observer.rs	18	17	94.44%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	14270
Covered Lines:	10071
Line Coverage:	70.57%
Coverage Strength:	59.65 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/unify-error-type (9704f19) with master (932d762)}