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/project/src/lsp_store.rs

@@ -3761,7 +3761,7 @@ impl LspStore {
             worktree_store,
             languages: languages.clone(),
             language_server_statuses: Default::default(),
-            nonce: StdRng::from_entropy().r#gen(),
+            nonce: StdRng::from_os_rng().random(),
             diagnostic_summaries: HashMap::default(),
             lsp_server_capabilities: HashMap::default(),
             lsp_document_colors: HashMap::default(),
@@ -3823,7 +3823,7 @@ impl LspStore {
             worktree_store,
             languages: languages.clone(),
             language_server_statuses: Default::default(),
-            nonce: StdRng::from_entropy().r#gen(),
+            nonce: StdRng::from_os_rng().random(),
             diagnostic_summaries: HashMap::default(),
             lsp_server_capabilities: HashMap::default(),
             lsp_document_colors: HashMap::default(),

crates/project/src/project_tests.rs

@@ -7661,7 +7661,7 @@ async fn test_staging_random_hunks(
         .unwrap_or(20);
 
     // Try to induce races between diff recalculation and index writes.
-    if rng.gen_bool(0.5) {
+    if rng.random_bool(0.5) {
         executor.deprioritize(*CALCULATE_DIFF_TASK);
     }
 
@@ -7717,7 +7717,7 @@ async fn test_staging_random_hunks(
     assert_eq!(hunks.len(), 6);
 
     for _i in 0..operations {
-        let hunk_ix = rng.gen_range(0..hunks.len());
+        let hunk_ix = rng.random_range(0..hunks.len());
         let hunk = &mut hunks[hunk_ix];
         let row = hunk.range.start.row;
 
@@ -7735,7 +7735,7 @@ async fn test_staging_random_hunks(
             hunk.secondary_status = SecondaryHunkAdditionPending;
         }
 
-        for _ in 0..rng.gen_range(0..10) {
+        for _ in 0..rng.random_range(0..10) {
             log::info!("yielding");
             cx.executor().simulate_random_delay().await;
         }