fix(rust-ci): pass explicit toolchain: stable to SHA-pinned dtolnay action

The reusable Rust CI bundle pins `dtolnay/rust-toolchain` by commit SHA
(`@67ef31d5…`). That action infers the toolchain from its `@`-ref (e.g.
`@stable`), but a SHA ref carries no version, so its "parse toolchain
version" step receives an empty `toolchain` and fails:

    'toolchain' is a required input
    Process completed with exit code 1

This is the estate-wide `rust-ci / Cargo check + clippy + fmt` red seen
on every PR across the ~28 repos consuming this reusable workflow
(confirmed live in iseriser run 28287559313).

Add `toolchain: stable` to all four `dtolnay/rust-toolchain` steps
(check, test, audit, coverage), matching the `# stable` pin comment.
Consumers inherit the fix on the next standards-reusable pin refresh.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A6PSzJWpRxtzGDjUCEh7Mx

Author: claude

.github/workflows/rust-ci-reusable.yml

@@ -149,8 +149,14 @@ jobs:
           ref: ${{ github.ref }}
 
       - name: Install Rust toolchain
+        # `toolchain:` is mandatory because the action is pinned by commit SHA:
+        # dtolnay/rust-toolchain infers the toolchain from the `@`-ref (e.g.
+        # `@stable`), but a SHA ref carries no version, so the action's "parse
+        # toolchain version" step fails with `'toolchain' is a required input`.
+        # See standards estate-wide rust-ci red.
         uses: dtolnay/rust-toolchain@67ef31d5b988238dd797d409d6f9574278e20537 # stable
         with:
+          toolchain: stable
           components: clippy, rustfmt
 
       - name: Cache cargo registry and build
@@ -190,7 +196,10 @@ jobs:
           ref: ${{ github.ref }}
 
       - name: Install Rust toolchain
+        # `toolchain:` mandatory under SHA pin — see Cargo check job above.
         uses: dtolnay/rust-toolchain@67ef31d5b988238dd797d409d6f9574278e20537 # stable
+        with:
+          toolchain: stable
 
       - name: Cache cargo registry and build
         uses: Swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4 # v2
@@ -230,7 +239,10 @@ jobs:
           ref: ${{ github.ref }}
 
       - name: Install Rust toolchain
+        # `toolchain:` mandatory under SHA pin — see Cargo check job above.
         uses: dtolnay/rust-toolchain@67ef31d5b988238dd797d409d6f9574278e20537 # stable
+        with:
+          toolchain: stable
 
       - name: Install cargo-audit
         # Use the binstall path when available to skip a from-source rebuild
@@ -260,7 +272,10 @@ jobs:
           ref: ${{ github.ref }}
 
       - name: Install Rust toolchain
+        # `toolchain:` mandatory under SHA pin — see Cargo check job above.
         uses: dtolnay/rust-toolchain@67ef31d5b988238dd797d409d6f9574278e20537 # stable
+        with:
+          toolchain: stable
 
       - name: Install tarpaulin
         run: cargo install cargo-tarpaulin --locked