server + ui: ping silent SSE streams every 1s and kick only after 3s so slow prefill never drops healthy connections (#25241)

* server + ui: ping silent SSE streams every 1s and kick only after 3s so slow prefill never drops healthy connections

* server + ui: sse_ping_interval becomes a per-request body field

Address review from ngxson: the global default returns to 30 so API
clients see no behavior change, and the WebUI sends sse_ping_interval: 1
in the request body since it owns the 3s visibility-kick contract and
declares the cadence it needs. Positive values keep the existing > 0
gate, -1 keeps its disabled semantics.

* server: move sse_ping_interval into the request schema

Address review from ngxson: the field is now a typed field_num with
hard limits (-1, INT32_MAX) bound to task_params, seeded from the CLI
default alongside the other inherited parameters. The raw json_value
read and its redundant comment are gone, and schema evaluation brings
type and range validation for free.

tools/server/server-task.h

@@ -54,6 +54,8 @@ struct task_params {
     bool return_tokens   = false;
     bool return_progress = false;
 
+    int32_t sse_ping_interval = 30; // seconds between SSE comment pings while the stream stays silent, -1 disables
+
     int32_t n_keep    =  0; // number of tokens to keep from initial prompt
     int32_t n_discard =  0; // number of tokens after n_keep that may be discarded when shifting context, 0 defaults to half
     int32_t n_predict = -1; // new tokens to predict