fix(security): truncate misaligned AES ciphertext instead of throwing

[ntedvs]

Summary

Fixes #28

aesDecrypt threw when ciphertext length wasn't a multiple of 16 bytes, crashing PDF.load() entirely for corrupted encrypted PDFs. Now truncates to the nearest block boundary and decrypts what's available.

aesDecryptWithIv keeps its strict validation since it's used for key derivation where misalignment is a real error.

Note

decryptObject in document-parser.ts still lacks a general try-catch for other decryption failures (e.g. PKCS#7 padding errors). That's a pre-existing gap, not introduced here, but worth addressing separately.

Test plan

• Truncation of misaligned ciphertext recovers valid data
• Sub-block-size ciphertext returns empty instead of throwing

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