Extract a scheduler crate from GPUI to enable unified integration testing of client and server code (#37326)

Extracts and cleans up GPUI's scheduler code into a new `scheduler`
crate, making it pluggable by external runtimes. This will enable
deterministic integration testing with cloud components by providing a
unified test scheduler across Zed and backend code. In Zed, it will
replace the existing GPUI scheduler for consistent async task management
across platforms.

## Changes

- **Core Implementation**: `TestScheduler` with seed-based
randomization, session tracking (`SessionId`), and foreground/background
task separation for reproducible testing.
- **Executors**: `ForegroundExecutor` (!Send, thread-local) and
`BackgroundExecutor` (Send, with blocking/timeout support) as
GPUI-compatible wrappers.
- **Clock and Timer**: Controllable `TestClock` and future-based `Timer`
for time-sensitive tests.
- **Testing APIs**: `once()`, `with_seed()`, and `many()` methods for
configurable test runs.
- **Dependencies**: Added `async-task`, `chrono`, `futures`, etc., with
updates to `Cargo.toml` and lock file.

## Benefits

- **Integration Testing**: Facilitates reliable async tests involving
cloud sessions, reducing flakiness via deterministic execution.
- **Pluggability**: Trait-based design (`Scheduler`) allows easy
integration into non-GPUI runtimes while maintaining GPUI compatibility.
- **Cleanup**: Refactors GPUI scheduler logic for clarity, correctness
(no `unwrap()`, proper error handling), and extensibility.

Follows Rust guidelines; run `./script/clippy` for verification.

- [x] Define and test a core scheduler that we think can power our cloud
code and GPUI
- [ ] Replace GPUI's scheduler


Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant_tools/src/edit_agent/create_file_parser.rs

@@ -204,7 +204,7 @@ mod tests {
     }
 
     fn parse_random_chunks(input: &str, parser: &mut CreateFileParser, rng: &mut StdRng) -> String {
-        let chunk_count = rng.gen_range(1..=cmp::min(input.len(), 50));
+        let chunk_count = rng.random_range(1..=cmp::min(input.len(), 50));
         let mut chunk_indices = (0..input.len()).choose_multiple(rng, chunk_count);
         chunk_indices.sort();
         chunk_indices.push(input.len());

crates/assistant_tools/src/edit_agent/edit_parser.rs

@@ -996,7 +996,7 @@ mod tests {
     }
 
     fn parse_random_chunks(input: &str, parser: &mut EditParser, rng: &mut StdRng) -> Vec<Edit> {
-        let chunk_count = rng.gen_range(1..=cmp::min(input.len(), 50));
+        let chunk_count = rng.random_range(1..=cmp::min(input.len(), 50));
         let mut chunk_indices = (0..input.len()).choose_multiple(rng, chunk_count);
         chunk_indices.sort();
         chunk_indices.push(input.len());

crates/assistant_tools/src/edit_agent/evals.rs

@@ -1399,7 +1399,7 @@ fn eval(
 }
 
 fn run_eval(eval: EvalInput, tx: mpsc::Sender<Result<EvalOutput>>) {
-    let dispatcher = gpui::TestDispatcher::new(StdRng::from_entropy());
+    let dispatcher = gpui::TestDispatcher::new(StdRng::from_os_rng());
     let mut cx = TestAppContext::build(dispatcher, None);
     let output = cx.executor().block_test(async {
         let test = EditAgentTest::new(&mut cx).await;
@@ -1707,7 +1707,7 @@ async fn retry_on_rate_limit<R>(mut request: impl AsyncFnMut() -> Result<R>) ->
         };
 
         if let Some(retry_after) = retry_delay {
-            let jitter = retry_after.mul_f64(rand::thread_rng().gen_range(0.0..1.0));
+            let jitter = retry_after.mul_f64(rand::rng().random_range(0.0..1.0));
             eprintln!("Attempt #{attempt}: Retry after {retry_after:?} + jitter of {jitter:?}");
             Timer::after(retry_after + jitter).await;
         } else {

crates/assistant_tools/src/edit_agent/streaming_fuzzy_matcher.rs

@@ -771,7 +771,7 @@ mod tests {
     }
 
     fn to_random_chunks(rng: &mut StdRng, input: &str) -> Vec<String> {
-        let chunk_count = rng.gen_range(1..=cmp::min(input.len(), 50));
+        let chunk_count = rng.random_range(1..=cmp::min(input.len(), 50));
         let mut chunk_indices = (0..input.len()).choose_multiple(rng, chunk_count);
         chunk_indices.sort();
         chunk_indices.push(input.len());