refactor: replace single-retry unresolved loop with fixed-point itera…

[Velli20]

…tion

Replace the single-pass retry in load_objects_with_decryption with a convergent fixed-point loop that iteratively resolves forward references (e.g. indirect /Length entries in stream dictionaries).

Changes:

• Extract try_load_object helper to deduplicate parse/decrypt/insert logic
• Iterate retries until all resolved, no progress, or 16-iteration cap
• Add UnresolvedObjects error variant with count, iterations, first_offset
• Add tests for indirect /Length resolution and unresolvable references

[Copilot]

Pull request overview

Refactors PdfReader object loading to resolve forward references (notably indirect /Length in streams) via a convergent fixed-point retry loop, improving robustness when xref iteration order encounters dependencies that haven’t been loaded yet.

Changes:

• Adds PdfReaderError::UnresolvedObjects { count, iterations, first_offset } for cases where forward references can’t be resolved after iterative retries.
• Replaces the prior single retry pass with an iterative retry loop capped at 16 iterations and stopping on no-progress convergence.
• Introduces try_load_object helper and adds tests covering indirect /Length resolution and unresolvable references.

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[Copilot]

test_unresolvable_reference_returns_error only asserts that the error string contains "failed to resolve", which would also pass for other error variants/messages. Consider matching on Err(PdfReaderError::UnresolvedObjects { .. }) (and optionally asserting count, iterations, and first_offset == obj4_offset) so the test reliably verifies the new error behavior.

Suggested change

	assert!(result.is_err(), "Should fail for unresolvable reference");
	let err_msg = match result {
	Err(e) => e.to_string(),
	Ok(_) => unreachable!(),
	};
	assert!(
	err_msg.contains("failed to resolve"),
	"Expected UnresolvedObjects error, got: {err_msg}"
	);
	match result {
	Err(super::PdfReaderError::UnresolvedObjects {
	count,
	iterations,
	first_offset,
	..
	}) => {
	assert_eq!(count, 1, "Expected one unresolved object");
	assert_eq!(iterations, 0, "Expected failure after one no-progress pass");
	assert_eq!(
	first_offset as usize,
	obj4_offset,
	"Expected unresolved object to be object 4"
	);
	}
	Err(e) => panic!("Expected UnresolvedObjects error, got: {e}"),
	Ok(_) => panic!("Should fail for unresolvable reference"),
	}