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.rs

@@ -1315,17 +1315,17 @@ mod tests {
 
     #[gpui::test(iterations = 100)]
     async fn test_random_indents(mut rng: StdRng) {
-        let len = rng.gen_range(1..=100);
+        let len = rng.random_range(1..=100);
         let new_text = util::RandomCharIter::new(&mut rng)
             .with_simple_text()
             .take(len)
             .collect::<String>();
         let new_text = new_text
             .split('\n')
-            .map(|line| format!("{}{}", " ".repeat(rng.gen_range(0..=8)), line))
+            .map(|line| format!("{}{}", " ".repeat(rng.random_range(0..=8)), line))
             .collect::<Vec<_>>()
             .join("\n");
-        let delta = IndentDelta::Spaces(rng.gen_range(-4..=4));
+        let delta = IndentDelta::Spaces(rng.random_range(-4i8..=4i8) as isize);
 
         let chunks = to_random_chunks(&mut rng, &new_text);
         let new_text_chunks = stream::iter(chunks.iter().enumerate().map(|(index, chunk)| {
@@ -1357,7 +1357,7 @@ 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());