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.

---------

Co-authored-by: Pascal <admin@serveurperso.com>

tools/server/server-common.cpp

@@ -667,7 +667,7 @@ void server_tokens::keep_first(size_t n) {
             // note that the case where we keep a full image at the end is allowed:
             //   tokens[n - 1] == LLAMA_TOKEN_NULL && tokens[n] != LLAMA_TOKEN_NULL
             if (tokens[n - 1] == LLAMA_TOKEN_NULL && tokens[n] == LLAMA_TOKEN_NULL) {
-                find_chunk(n - 1); // will throw an error if the token is not begin-of-chunk
+                find_chunk(n); // will throw an error if the cut is not at a chunk boundary
             }
         }
         // remove all image chunks that are not used anymore