server: validate n_batch == n_ubatch for embeddings (#6263)

Fixes #6263 where server accepts mismatched batch/ubatch values with
embeddings, leading to suboptimal or incorrect behavior.

Problem: Embeddings and reranking use non-causal attention which requires
all tokens to be processed within a single ubatch. When n_batch != n_ubatch,
the configuration is incoherent. Default values differ (n_batch=2048,
n_ubatch=512), so users encounter this frequently.

Solution:
- Add parameter validation in main() after common_params_parse()
- When embeddings enabled and n_batch != n_ubatch:
  * Log warnings explaining the requirement
  * Automatically set both to min(n_batch, n_ubatch)
  * Ensure coherent configuration

This follows the auto-correction approach suggested by @mirekphd
and provides better UX than strict rejection.

Testing:
✅ Builds successfully
✅ Validation triggers: -b 2048 -ub 512 --embedding → logs warnings, adjusts both to 512
✅ No false positives: -b 512 -ub 512 --embedding → no warnings
✅ Verified on macOS M3 Pro with embedding model

Author: ytian218

tools/server/server.cpp

@@ -3657,6 +3657,17 @@ int main(int argc, char ** argv) {
         return 1;
     }
 
+    // validate batch size for embeddings and reranking
+    // non-causal attention (embeddings/reranking) requires n_batch == n_ubatch
+    // see https://github.com/ggml-org/llama.cpp/issues/6263
+    if (params.embedding && params.n_batch != params.n_ubatch) {
+        LOG_WRN("%s: embeddings/reranking mode requires n_batch == n_ubatch\n", __func__);
+        LOG_WRN("%s: setting both to min(%d, %d) = %d to avoid configuration issues\n",
+                __func__, params.n_batch, params.n_ubatch,
+                std::min(params.n_batch, params.n_ubatch));
+        params.n_batch = params.n_ubatch = std::min(params.n_batch, params.n_ubatch);
+    }
+
     // TODO: should we have a separate n_parallel parameter for the server?
     //       https://github.com/ggml-org/llama.cpp/pull/16736#discussion_r2483763177
     // TODO: this is a common configuration that is suitable for most local use cases