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/network.rs

@@ -65,8 +65,8 @@ impl<T: Clone, R: rand::Rng> Network<T, R> {
                 for message in &messages {
                     // Insert one or more duplicates of this message, potentially *before* the previous
                     // message sent by this peer to simulate out-of-order delivery.
-                    for _ in 0..self.rng.gen_range(1..4) {
-                        let insertion_index = self.rng.gen_range(0..inbox.len() + 1);
+                    for _ in 0..self.rng.random_range(1..4) {
+                        let insertion_index = self.rng.random_range(0..inbox.len() + 1);
                         inbox.insert(
                             insertion_index,
                             Envelope {
@@ -85,7 +85,7 @@ impl<T: Clone, R: rand::Rng> Network<T, R> {
 
     pub fn receive(&mut self, receiver: ReplicaId) -> Vec<T> {
         let inbox = self.inboxes.get_mut(&receiver).unwrap();
-        let count = self.rng.gen_range(0..inbox.len() + 1);
+        let count = self.rng.random_range(0..inbox.len() + 1);
         inbox
             .drain(0..count)
             .map(|envelope| envelope.message)