Stdlib: gzcompress/gzencode/gzdeflate null $level E_DEPRECATED (#31445)

[PurHur]

Summary

• Close Regression: gzcompress/gzencode/gzdeflate(..., null) level silent — Zend E_DEPRECATED (ext/zlib/zlib.c) #31445: gzcompress / gzencode / gzdeflate with null $level now emit Zend-identical E_DEPRECATED then coerce to 0 (still compresses).
• VM: VmZlibArg::coerceInt / encoding null path via VmNullNumberParamDeprecation.
• JIT: JitStrictIntArg soft-null DEP + coerce (compile-time null and boxed null).
• declare(strict_types=1) still TypeErrors (unchanged).
• php-src ref: ext/zlib/zlib.c / zlib.stub.php (int $level = -1).

Test plan

• ./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_gz_null_level.php'
• ./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_gz_null_level.php'
• strict TypeError repro (VM + JIT)
• ./script/phpunit.sh --filter 'Issue31445GzNullLevelTest|GzNullLevelSoft' → OK (6 tests)

Verification transcript (VM soft-null)

ERR[8192]: gzcompress(): Passing null to parameter #2 ($level) of type int is deprecated
gzcompress OK
ERR[8192]: gzencode(): Passing null to parameter #2 ($level) of type int is deprecated
gzencode OK
ERR[8192]: gzdeflate(): Passing null to parameter #2 ($level) of type int is deprecated
gzdeflate OK

Closes #31445

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