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/agent_ui/src/buffer_codegen.rs

@@ -1139,7 +1139,7 @@ mod tests {
         );
         while !new_text.is_empty() {
             let max_len = cmp::min(new_text.len(), 10);
-            let len = rng.gen_range(1..=max_len);
+            let len = rng.random_range(1..=max_len);
             let (chunk, suffix) = new_text.split_at(len);
             chunks_tx.unbounded_send(chunk.to_string()).unwrap();
             new_text = suffix;
@@ -1208,7 +1208,7 @@ mod tests {
         );
         while !new_text.is_empty() {
             let max_len = cmp::min(new_text.len(), 10);
-            let len = rng.gen_range(1..=max_len);
+            let len = rng.random_range(1..=max_len);
             let (chunk, suffix) = new_text.split_at(len);
             chunks_tx.unbounded_send(chunk.to_string()).unwrap();
             new_text = suffix;
@@ -1277,7 +1277,7 @@ mod tests {
         );
         while !new_text.is_empty() {
             let max_len = cmp::min(new_text.len(), 10);
-            let len = rng.gen_range(1..=max_len);
+            let len = rng.random_range(1..=max_len);
             let (chunk, suffix) = new_text.split_at(len);
             chunks_tx.unbounded_send(chunk.to_string()).unwrap();
             new_text = suffix;