Update Rust crate slab to 0.4.9 (#27480)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [slab](https://github.com/tokio-rs/slab) | workspace.dependencies | patch | `0.4.8` -> `0.4.9` |

---

### Configuration

📅 **Schedule**: Branch creation - "on the first day of the month" in timezone America/Los_Angeles, Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by [Mend Renovate](https://www.mend.io/free-developer-tools/renovate/). View repository job log [here](https://developer.mend.io/github/get-convex/convex).

GitOrigin-RevId: 459d1cb41bee7528751b6d632e568c6d0bd19877

Author: renovate[bot]

Cargo.toml

@@ -112,7 +112,7 @@ serde = { version = "1", features = [ "derive" ] }
 serde_json = { version = "1", features = [ "float_roundtrip", "preserve_order" ] }
 sha1 = { version = "0.10.5", features = [ "oid" ] }
 sha2 = { version = "0.10.8" }
-slab = "0.4.8"
+slab = "0.4.9"
 sodiumoxide = "^0.2"
 sourcemap = "8"
 strum = { version = "0.26", features = [ "derive" ] }