fix: truncate markdown image name context to prevent OS filename limit

[gdccyuen]

What

When last_context (the markdown text preceding an image) is very long, the generated image filename exceeds the OS 255-byte limit, causing os.rename to fail with OSError: [Errno 36] File name too long.

Error

OSError: [Errno 36] File name too long:
'/tmp/.../images/a68d4c...jpg' ->
'/tmp/.../images/image-1-## 5. How to determine whether an application falls within or beyond the _$500,000 threshold For EE A, whose employment contract is terminated by 1-month notice ending on 30 April 2025, the relevant date of termination of employment falls on 30 April 2025...jpg'

Fix

Truncate image_name_context to 60 characters after sanitization in build_markdown_image_name(). The context is cosmetic (used in the filename for debuggability), so truncation has no functional impact.

Verification

Tested end-to-end with a 44-page PDF that triggered the original error — parsing succeeds after this fix.