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/editor/src/display_map/tab_map.rs

@@ -736,9 +736,9 @@ mod tests {
 
     #[gpui::test(iterations = 100)]
     fn test_random_tabs(cx: &mut gpui::App, mut rng: StdRng) {
-        let tab_size = NonZeroU32::new(rng.gen_range(1..=4)).unwrap();
-        let len = rng.gen_range(0..30);
-        let buffer = if rng.r#gen() {
+        let tab_size = NonZeroU32::new(rng.random_range(1..=4)).unwrap();
+        let len = rng.random_range(0..30);
+        let buffer = if rng.random() {
             let text = util::RandomCharIter::new(&mut rng)
                 .take(len)
                 .collect::<String>();
@@ -769,11 +769,11 @@ mod tests {
         );
 
         for _ in 0..5 {
-            let end_row = rng.gen_range(0..=text.max_point().row);
-            let end_column = rng.gen_range(0..=text.line_len(end_row));
+            let end_row = rng.random_range(0..=text.max_point().row);
+            let end_column = rng.random_range(0..=text.line_len(end_row));
             let mut end = TabPoint(text.clip_point(Point::new(end_row, end_column), Bias::Right));
-            let start_row = rng.gen_range(0..=text.max_point().row);
-            let start_column = rng.gen_range(0..=text.line_len(start_row));
+            let start_row = rng.random_range(0..=text.max_point().row);
+            let start_column = rng.random_range(0..=text.line_len(start_row));
             let mut start =
                 TabPoint(text.clip_point(Point::new(start_row, start_column), Bias::Left));
             if start > end {