Guard low-memory parsing by bounding Flate decode and image payload retention

[vitormattos]

Summary

• derive a conservative effective Flate decode cap when decodeMemoryLimit is not explicitly configured
• apply the same cap to the compress.zlib fallback path
• skip retaining image stream payloads when PHP memory is constrained to avoid cumulative OOM during parsing
• add focused regression coverage in DocumentIssueFocusTest for PullRequest457.pdf

Reproduction

Before this patch, the command below fails with fatal OOM at memory_limit=128M:

docker run --rm -v "$PWD":/work -w /work smalot-pdf-bench:local \
  php -d memory_limit=128M -r 'require "vendor/autoload.php"; $parser = new Smalot\\PdfParser\\Parser(); $doc = $parser->parseFile("samples/bugs/PullRequest457.pdf"); echo count($doc->getPages()), PHP_EOL;'

After this patch it prints 28.

Tests

• make run-phpunit ARGS="tests/PHPUnit/Integration/DocumentIssueFocusTest.php --filter testParseFileWithLargeFlateStreams"
• make run-phpunit ARGS="tests/PHPUnit/Integration/PageTest.php --filter testGetTextPullRequest457"

PDF Source

• File: samples/bugs/PullRequest457.pdf
• Source URL: N/A (existing repository fixture reused for this fix)

[vitormattos]

Superseded by the RawDataParser consolidation chain in the fork.

This fix (flate decode memory guard, MemoryLimit helper, test stabilization) is included in vitormattos#31, stacked directly on the fix/invalid-object-reference-tolerant-parser consolidation branch.

Supersedes #801