server: reject partial media truncation (#24076)
* server: reject partial media truncation
* server: keep only the keep_first fix
Drop the mtmd test helper change, which no longer builds since
clip_image_f32_batch stores its entries by value, and drop the
vision test: no test fixture reaches a cut between two adjacent
media chunks with a reused cache (tinygemma3 uses SWA and wraps
images in text tokens, tinyopenjev and small-test are recurrent),
so the test passed or failed independently of the fix.
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Co-authored-by: Pascal <admin@serveurperso.com>
