server : support slot save/restore with media inputs (#26640)

* server : save serialized image chunks at the end of the llama state

* server : support multimodal slot state save/restore with packed payload

* server : refine image slot state serialization

* server : support media slot state and centralize media validation

* server : remove unnecessary comment

* server : remove defensive media checks and move the chunk type check to validate()

tools/server/server-common.h

@@ -201,11 +201,14 @@ public:
     // for compatibility with context shift and prompt truncation
     void insert(const llama_tokens & inp_tokens);
 
-    // for compatibility with speculative decoding, ctx shift, slot save/load
+    // for compatibility with speculative decoding, ctx shift
     const llama_tokens & get_tokens() const;
 
     llama_tokens get_text_tokens() const;
 
+    std::vector<char> serialize() const;
+    static server_tokens deserialize(const llama_tokens & packed, bool has_mtmd);
+
     // for compatibility with speculative decoding
     void set_token(llama_pos pos, llama_token id);
 
@@ -213,9 +216,6 @@ public:
 
     bool empty() const { return tokens.empty(); }
 
-    // true if the sequence actually contains image/audio chunks.
-    bool has_media() const { return !map_idx_to_media.empty(); }
-
     void clear() {
         map_idx_to_media.clear();
         tokens.clear();
@@ -230,7 +230,7 @@ public:
     // split the tokens into message spans, skipping over media chunks
     common_chat_msg_spans find_message_spans(const common_chat_msg_delimiters & delims) const;
 
-    // make sure all text tokens are within the vocab range
+    // check text token IDs and the mapping between media chunks and token ranges
     bool validate(const struct llama_context * ctx) const;
 
     server_tokens clone() const;