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/text/src/text.rs

@@ -1818,8 +1818,8 @@ impl Buffer {
     }
 
     pub fn random_byte_range(&self, start_offset: usize, rng: &mut impl rand::Rng) -> Range<usize> {
-        let end = self.clip_offset(rng.gen_range(start_offset..=self.len()), Bias::Right);
-        let start = self.clip_offset(rng.gen_range(start_offset..=end), Bias::Right);
+        let end = self.clip_offset(rng.random_range(start_offset..=self.len()), Bias::Right);
+        let start = self.clip_offset(rng.random_range(start_offset..=end), Bias::Right);
         start..end
     }
 
@@ -1841,7 +1841,7 @@ impl Buffer {
             let range = self.random_byte_range(new_start, rng);
             last_end = Some(range.end);
 
-            let new_text_len = rng.gen_range(0..10);
+            let new_text_len = rng.random_range(0..10);
             let new_text: String = RandomCharIter::new(&mut *rng).take(new_text_len).collect();
 
             edits.push((range, new_text.into()));
@@ -1877,7 +1877,7 @@ impl Buffer {
         use rand::prelude::*;
 
         let mut ops = Vec::new();
-        for _ in 0..rng.gen_range(1..=5) {
+        for _ in 0..rng.random_range(1..=5) {
             if let Some(entry) = self.history.undo_stack.choose(rng) {
                 let transaction = entry.transaction.clone();
                 log::info!(