fix: aggregate multi-space object header support

[vitormattos]

Summary

• Mirror of upstream fix for malformed indirect object headers with extra whitespace.
• Keeps aggregate branch aligned while upstream review runs.

Upstream PR

• fix: support multi-space object headers smalot/pdfparser#800

Scope

• Branch: fix/nearby-object-header-fallback
• Target aggregate branch: fork/libresign-parser-fixes

PDF Source

• Corpus: veraPDF-corpus
• Fixture origin: Isartor test files/PDFA-1b/6.1 File structure/6.1.8 Indirect objects/isartor-6-1-8-t01-fail-a.pdf
• Source URL: https://github.com/veraPDF/veraPDF-corpus/blob/staging/Isartor%20test%20files/PDFA-1b/6.1%20File%20structure/6.1.8%20Indirect%20objects/isartor-6-1-8-t01-fail-a.pdf
• Local copy used in tests: samples/bugs/PullRequestNearbyObjectHeaderOffset.pdf