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/rpc/src/auth.rs

@@ -1,6 +1,6 @@
 use anyhow::{Context as _, Result};
 use base64::prelude::*;
-use rand::{Rng as _, thread_rng};
+use rand::prelude::*;
 use rsa::pkcs1::{DecodeRsaPublicKey, EncodeRsaPublicKey};
 use rsa::traits::PaddingScheme;
 use rsa::{Oaep, Pkcs1v15Encrypt, RsaPrivateKey, RsaPublicKey};
@@ -31,7 +31,7 @@ pub struct PrivateKey(RsaPrivateKey);
 
 /// Generate a public and private key for asymmetric encryption.
 pub fn keypair() -> Result<(PublicKey, PrivateKey)> {
-    let mut rng = thread_rng();
+    let mut rng = RsaRngCompat::new();
     let bits = 2048;
     let private_key = RsaPrivateKey::new(&mut rng, bits)?;
     let public_key = RsaPublicKey::from(&private_key);
@@ -40,10 +40,10 @@ pub fn keypair() -> Result<(PublicKey, PrivateKey)> {
 
 /// Generate a random 64-character base64 string.
 pub fn random_token() -> String {
-    let mut rng = thread_rng();
+    let mut rng = rand::rng();
     let mut token_bytes = [0; 48];
     for byte in token_bytes.iter_mut() {
-        *byte = rng.r#gen();
+        *byte = rng.random();
     }
     BASE64_URL_SAFE.encode(token_bytes)
 }
@@ -52,7 +52,7 @@ impl PublicKey {
     /// Convert a string to a base64-encoded string that can only be decoded with the corresponding
     /// private key.
     pub fn encrypt_string(&self, string: &str, format: EncryptionFormat) -> Result<String> {
-        let mut rng = thread_rng();
+        let mut rng = RsaRngCompat::new();
         let bytes = string.as_bytes();
         let encrypted_bytes = match format {
             EncryptionFormat::V0 => self.0.encrypt(&mut rng, Pkcs1v15Encrypt, bytes),
@@ -107,6 +107,36 @@ impl TryFrom<String> for PublicKey {
     }
 }
 
+// TODO: remove once we rsa v0.10 is released.
+struct RsaRngCompat(rand::rngs::ThreadRng);
+
+impl RsaRngCompat {
+    fn new() -> Self {
+        Self(rand::rng())
+    }
+}
+
+impl rsa::signature::rand_core::RngCore for RsaRngCompat {
+    fn next_u32(&mut self) -> u32 {
+        self.0.next_u32()
+    }
+
+    fn next_u64(&mut self) -> u64 {
+        self.0.next_u64()
+    }
+
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        self.0.fill_bytes(dest);
+    }
+
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rsa::signature::rand_core::Error> {
+        self.fill_bytes(dest);
+        Ok(())
+    }
+}
+
+impl rsa::signature::rand_core::CryptoRng for RsaRngCompat {}
+
 #[cfg(test)]
 mod tests {
     use super::*;