fix: Run tests single-threaded to avoid env var race conditions

- Update pre-commit hooks to use --test-threads=1
- Update CI workflow to run tests single-threaded
- This prevents test failures due to concurrent environment variable modifications

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.github/workflows/ci.yml

@@ -73,9 +73,9 @@ jobs:
     - name: Build
       run: cargo build --verbose
     - name: Run unit tests
-      run: cargo test --verbose
+      run: cargo test --verbose -- --test-threads=1
     - name: Run e2e tests
-      run: cargo test --test '*' --verbose
+      run: cargo test --test '*' --verbose -- --test-threads=1
 
   fmt:
     name: Rustfmt

.pre-commit-config.yaml

@@ -25,14 +25,14 @@ repos:
 
       - id: cargo-test
         name: cargo test
-        entry: cargo test
+        entry: cargo test -- --test-threads=1
         language: system
         types: [rust]
         pass_filenames: false
 
       - id: cargo-test-e2e
         name: cargo e2e tests
-        entry: cargo test --test '*'
+        entry: cargo test --test '*' -- --test-threads=1
         language: system
         types: [rust]
         pass_filenames: false