fix: resolve all clippy --all-targets warnings across 15 files

- gateway/mod.rs: move send_json before test module (items_after_test_module)
- memory/vector.rs: fix float_cmp, cast_precision_loss, approx_constant
- memory/chunker.rs: fix format_collect, format_push_string, write_with_newline
- memory/sqlite.rs: fix useless_vec
- heartbeat/engine.rs: fix format_collect, write_with_newline
- config/schema.rs: fix needless_raw_string_hashes
- tools/composio.rs: fix needless_raw_string_hashes
- integrations/registry.rs: fix uninlined_format_args, unused import
- tunnel/mod.rs: fix doc_markdown
- skills/mod.rs: allow similar_names in test module
- channels/cli.rs: fix unreadable_literal
- observability/mod.rs: fix manual_string_new
- runtime/mod.rs: fix manual_string_new
- examples/custom_memory.rs: add Default impl (new_without_default)
- examples/custom_channel.rs: fix needless_borrows_for_generic_args

src/heartbeat/engine.rs

@@ -193,7 +193,11 @@ mod tests {
 
     #[test]
     fn parse_tasks_many_tasks() {
-        let content: String = (0..100).map(|i| format!("- Task {i}\n")).collect();
+        let content: String = (0..100).fold(String::new(), |mut s, i| {
+            use std::fmt::Write;
+            let _ = writeln!(s, "- Task {i}");
+            s
+        });
         let tasks = HeartbeatEngine::parse_tasks(&content);
         assert_eq!(tasks.len(), 100);
         assert_eq!(tasks[99], "Task 99");