fix: [ci] Use Cargo.toml instead of Cargo.lock for cache key

Cargo.lock is gitignored (library crate convention), so hashFiles
fails on CI. Use Cargo.toml hash instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: iglocska

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
             ~/.cargo/registry
             ~/.cargo/git
             target
-          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.toml') }}
           restore-keys: ${{ runner.os }}-cargo-
 
       - name: Build
@@ -82,7 +82,7 @@ jobs:
             ~/.cargo/registry
             ~/.cargo/git
             target
-          key: ${{ runner.os }}-cargo-integ-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-integ-${{ hashFiles('**/Cargo.toml') }}
           restore-keys: ${{ runner.os }}-cargo-integ-
 
       - name: Build integration tests