build(deps): Bump rust-toolchain

Bumps [rust-toolchain](https://github.com/rust-lang/rust) from nightly-2025-09-01 to nightly-2025-09-15.
- [Release notes](https://github.com/rust-lang/rust/releases)
- [Changelog](https://github.com/rust-lang/rust/blob/master/RELEASES.md)
- [Commits](https://github.com/rust-lang/rust/commits)

---
updated-dependencies:
- dependency-name: rust-toolchain
  dependency-version: nightly-2025-09-15
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

rust-toolchain.toml

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "nightly-2025-09-01"
+channel = "nightly-2025-09-15"
 components = [
     "llvm-tools",
     "rust-src",