perf(index-node): make proofOfIndexing resolver async

Remove synchronous `block_on` call in `resolve_proof_of_indexing` which
was blocking tokio worker threads while waiting for database queries.

Before this change, each POI query blocked an entire tokio worker thread.

After this change, POI queries properly yield to the async runtime while
waiting for database I/O, allowing the connection pool to be fully utilized.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MoonBoi9001

server/index-node/src/resolver.rs

@@ -352,7 +352,10 @@ where
         ))
     }
 
-    fn resolve_proof_of_indexing(&self, field: &a::Field) -> Result<r::Value, QueryExecutionError> {
+    async fn resolve_proof_of_indexing(
+        &self,
+        field: &a::Field,
+    ) -> Result<r::Value, QueryExecutionError> {
         let deployment_id = field
             .get_required::<DeploymentHash>("subgraph")
             .expect("Valid subgraphId required");
@@ -381,7 +384,7 @@ where
         let poi_fut = self
             .store
             .get_proof_of_indexing(&deployment_id, &indexer, block.clone());
-        let poi = match graph::futures03::executor::block_on(poi_fut) {
+        let poi = match poi_fut.await {
             Ok(Some(poi)) => r::Value::String(format!("0x{}", hex::encode(poi))),
             Ok(None) => r::Value::Null,
             Err(e) => {
@@ -791,7 +794,7 @@ where
             field.name.as_str(),
             scalar_type.name.as_str(),
         ) {
-            ("Query", "proofOfIndexing", "Bytes") => self.resolve_proof_of_indexing(field),
+            ("Query", "proofOfIndexing", "Bytes") => self.resolve_proof_of_indexing(field).await,
             ("Query", "blockData", "JSONObject") => self.resolve_block_data(field).await,
             ("Query", "blockHashFromNumber", "Bytes") => {
                 self.resolve_block_hash_from_number(field).await