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>

Cargo.lock

@@ -26,7 +26,7 @@ dependencies = [
  "portable-pty",
  "project",
  "prompt_store",
- "rand 0.8.5",
+ "rand 0.9.1",
  "serde",
  "serde_json",
  "settings",
@@ -79,7 +79,7 @@ dependencies = [
  "log",
  "pretty_assertions",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "serde_json",
  "settings",
  "text",
@@ -172,7 +172,7 @@ dependencies = [
  "pretty_assertions",
  "project",
  "prompt_store",
- "rand 0.8.5",
+ "rand 0.9.1",
  "ref-cast",
  "rope",
  "schemars",
@@ -408,7 +408,7 @@ dependencies = [
  "project",
  "prompt_store",
  "proto",
- "rand 0.8.5",
+ "rand 0.9.1",
  "release_channel",
  "rope",
  "rules_library",
@@ -834,7 +834,7 @@ dependencies = [
  "project",
  "prompt_store",
  "proto",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "rpc",
  "serde",
@@ -933,7 +933,7 @@ dependencies = [
  "parking_lot",
  "pretty_assertions",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "serde",
  "serde_json",
@@ -985,7 +985,7 @@ dependencies = [
  "pretty_assertions",
  "project",
  "prompt_store",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "reqwest_client",
  "rust-embed",
@@ -2478,7 +2478,7 @@ dependencies = [
  "language",
  "log",
  "pretty_assertions",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rope",
  "serde_json",
  "sum_tree",
@@ -2899,7 +2899,7 @@ dependencies = [
  "language",
  "log",
  "postage",
- "rand 0.8.5",
+ "rand 0.9.1",
  "release_channel",
  "rpc",
  "settings",
@@ -3086,7 +3086,7 @@ dependencies = [
  "parking_lot",
  "paths",
  "postage",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "release_channel",
  "rpc",
@@ -3335,7 +3335,7 @@ dependencies = [
  "prometheus",
  "prompt_store",
  "prost 0.9.0",
- "rand 0.8.5",
+ "rand 0.9.1",
  "recent_projects",
  "release_channel",
  "remote",
@@ -4697,7 +4697,7 @@ dependencies = [
  "markdown",
  "pretty_assertions",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "serde",
  "serde_json",
  "settings",
@@ -5068,7 +5068,7 @@ dependencies = [
  "parking_lot",
  "pretty_assertions",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "release_channel",
  "rpc",
@@ -5563,7 +5563,7 @@ dependencies = [
  "parking_lot",
  "paths",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "release_channel",
  "remote",
  "reqwest_client",
@@ -6412,7 +6412,7 @@ dependencies = [
  "log",
  "parking_lot",
  "pretty_assertions",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "rope",
  "schemars",
@@ -7465,7 +7465,7 @@ dependencies = [
  "pathfinder_geometry",
  "postage",
  "profiling",
- "rand 0.8.5",
+ "rand 0.9.1",
  "raw-window-handle",
  "refineable",
  "reqwest_client",
@@ -9078,7 +9078,7 @@ dependencies = [
  "parking_lot",
  "postage",
  "pretty_assertions",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "rpc",
  "schemars",
@@ -10392,7 +10392,7 @@ dependencies = [
  "parking_lot",
  "pretty_assertions",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rope",
  "serde",
  "settings",
@@ -12618,7 +12618,7 @@ dependencies = [
  "postage",
  "prettier",
  "pretty_assertions",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "release_channel",
  "remote",
@@ -13892,7 +13892,7 @@ dependencies = [
  "ctor",
  "gpui",
  "log",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rayon",
  "smallvec",
  "sum_tree",
@@ -13921,7 +13921,7 @@ dependencies = [
  "gpui",
  "parking_lot",
  "proto",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rsa",
  "serde",
  "serde_json",
@@ -14356,6 +14356,19 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "scheduler"
+version = "0.1.0"
+dependencies = [
+ "async-task",
+ "chrono",
+ "futures 0.3.31",
+ "parking",
+ "parking_lot",
+ "rand 0.9.1",
+ "workspace-hack",
+]
+
 [[package]]
 name = "schema_generator"
 version = "0.1.0"
@@ -15655,7 +15668,7 @@ name = "streaming_diff"
 version = "0.1.0"
 dependencies = [
  "ordered-float 2.10.1",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rope",
  "util",
  "workspace-hack",
@@ -15769,7 +15782,7 @@ dependencies = [
  "arrayvec",
  "ctor",
  "log",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rayon",
  "workspace-hack",
  "zlog",
@@ -16360,7 +16373,7 @@ dependencies = [
  "futures 0.3.31",
  "gpui",
  "libc",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "release_channel",
  "schemars",
@@ -16408,7 +16421,7 @@ dependencies = [
  "language",
  "log",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "schemars",
  "search",
@@ -16440,7 +16453,7 @@ dependencies = [
  "log",
  "parking_lot",
  "postage",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "rope",
  "smallvec",
@@ -17797,7 +17810,7 @@ dependencies = [
  "libc",
  "log",
  "nix 0.29.0",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "rust-embed",
  "schemars",
@@ -18588,7 +18601,7 @@ dependencies = [
  "futures 0.3.31",
  "gpui",
  "parking_lot",
- "rand 0.8.5",
+ "rand 0.9.1",
  "workspace-hack",
  "zlog",
 ]
@@ -20047,7 +20060,7 @@ dependencies = [
  "paths",
  "postage",
  "pretty_assertions",
- "rand 0.8.5",
+ "rand 0.9.1",
  "rpc",
  "schemars",
  "serde",
@@ -20812,7 +20825,7 @@ dependencies = [
  "menu",
  "postage",
  "project",
- "rand 0.8.5",
+ "rand 0.9.1",
  "regex",
  "release_channel",
  "reqwest_client",