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/git/blame.rs

@@ -1107,7 +1107,7 @@ mod tests {
         init_test(cx);
 
         let fs = FakeFs::new(cx.executor());
-        let buffer_initial_text_len = rng.gen_range(5..15);
+        let buffer_initial_text_len = rng.random_range(5..15);
         let mut buffer_initial_text = Rope::from(
             RandomCharIter::new(&mut rng)
                 .take(buffer_initial_text_len)
@@ -1159,7 +1159,7 @@ mod tests {
         git_blame.update(cx, |blame, cx| blame.check_invariants(cx));
 
         for _ in 0..operations {
-            match rng.gen_range(0..100) {
+            match rng.random_range(0..100) {
                 0..=19 => {
                     log::info!("quiescing");
                     cx.executor().run_until_parked();
@@ -1202,8 +1202,8 @@ mod tests {
         let mut blame_entries = Vec::new();
         for ix in 0..5 {
             if last_row < max_row {
-                let row_start = rng.gen_range(last_row..max_row);
-                let row_end = rng.gen_range(row_start + 1..cmp::min(row_start + 3, max_row) + 1);
+                let row_start = rng.random_range(last_row..max_row);
+                let row_end = rng.random_range(row_start + 1..cmp::min(row_start + 3, max_row) + 1);
                 blame_entries.push(blame_entry(&ix.to_string(), row_start..row_end));
                 last_row = row_end;
             } else {