fix(rawdata): recover malformed xref_command_missing startxref path

[vitormattos]

Bug fixed in this PR

Some malformed PDFs contain an unusable startxref target and/or a missing usable xref command near the advertised offset.
In this scenario, parsing failed with Unable to find xref.

This PR improves RawDataParser recovery by:

• accepting subsection-start offsets in decodeXref (not only exact xref keyword offsets),
• probing a nearby xref anchor before deciding decode path,
• recovering xref/trailer data from nearby trailer/object headers when startxref is missing or unusable,
• backfilling missing xref offsets from object headers when trailer references are incomplete.

Fixture(s) and source

• samples/bugs/rawdata/PullRequest815-xref-command-missing.pdf
  • Source corpus file: dev-tools/pdf-dataset/pdfs/pdfjs/xref_command_missing.pdf
  • Upstream source: https://github.com/mozilla/pdf.js/blob/master/test/pdfs/xref_command_missing.pdf

Tests

Focused gate executed with filter:

• RawDataParserTest::testParseRawDataIssuePullRequest815XrefCommandMissing

Result:

• OK (1 test, 1 assertion)

Note to maintainer

You can merge this PR directly, or use it as a reviewable base fix and merge through the integration branch/aggregator flow.

[vitormattos]

Closing as duplicate/superseded: this fix was incorporated into PR #796 (branch fix/invalid-object-reference-tolerant-parser), including regression test and fixture, to avoid parallel PRs touching RawDataParser.