feat(ai-agent): Phase 3 — explain_query tool, parallel/sequential execution, DB gating (#1187)

- Add explain_query tool with structured EXPLAIN result, gated on supports_explain_plan()
- Add supports_sql_query() to gate execute_query/get_sample_data for non-SQL DBs (Redis, ES, MongoDB, etc.)
- Split agent loop tool dispatch: execute_query runs sequentially, all others run in parallel via join_all
- Wire explain_query result button to ExplainPlanViewer in AiAssistant + App.vue
- Fix O(n²) agentSteps rebuild: incremental O(1) push per event during streaming
- finally block falls back to full rebuild only when streaming push did not occur

crates/dbx-core/src/query_execution_sql.rs

@@ -79,6 +79,20 @@ pub fn supports_explain_plan(database_type: Option<DatabaseType>) -> bool {
     )
 }
 
+/// Returns true for databases that support SQL query execution (execute_query / get_sample_data).
+/// Non-SQL databases (Redis, MongoDB, Elasticsearch, InfluxDB, Neo4j, etcd) are excluded.
+pub fn supports_sql_query(database_type: DatabaseType) -> bool {
+    !matches!(
+        database_type,
+        DatabaseType::Redis
+            | DatabaseType::MongoDb
+            | DatabaseType::Elasticsearch
+            | DatabaseType::InfluxDb
+            | DatabaseType::Neo4j
+            | DatabaseType::Etcd
+    )
+}
+
 pub fn is_safe_dameng_autotrace_sql(sql: &str) -> bool {
     let source = strip_trailing_semicolons(sql.trim());
     if source.is_empty() || has_extra_statement_after_semicolon(&source) {