mtmd: add chunk save/load function

[ngxson]

Overview

Add a save/load API call for mtmd_input_chunk, ref: #26640 (review)

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: 30% code (most repetitive changes) are AI-written

[ggerganov]

Using MTMD_INPUT_CHUNK_TYPE_COUNT would be more consistent with the rest of the codebase.

[CHIPMUNK-T0T]

@ngxson
Thanks for working on this API for #26640.

This is a much cleaner and more general foundation than adding cache-only fields to mtmd_input_chunk. I agree that chunk serialization and deserialization belong in the mtmd API proper.

Once this API is merged, I will update #26640 to use mtmd_input_chunk_save() / mtmd_input_chunk_load() and drop the cache-only fields and mtmd_input_chunk_init_image_cache() added there.

The slot-specific integration will remain in #26640: the chunk positions, the outer file framing, the consistency checks against the restored token sequence, and the server-side tests.