Detect tools changes in smithy-rs since the last update to config.json (#1382)

## Motivation and Context
When modifying the
[tools](https://github.com/smithy-lang/smithy-rs/tree/main/tools)
directory in `smithy-rs`, we must update our production release
infrastructure with a new container image tag before updating
`smithyRsVersion` in `config.json`. Failing to do so causes production
build failures.

For instance, during MSRV updates, we modify the
[Dockerfile](https://github.com/smithy-lang/smithy-rs/blob/main/tools/ci-build/Dockerfile)
in the `tools` directory. If we update `smithyRsVersion` in
`config.json` without first updating the container image tag in
production release environment, builds will fail because the code
generator emits Rust code compliant with the new MSRV, but the container
still uses an older compiler for testing.

This PR adds a mechanism to prevent this operational error by:
1. Running git diff between release tags (current `smithyRsVersion` in
main vs. PR branch)
2. Checking for any changes under the tools directory
3. Failing the release check workflow if changes are detected

Once the image tag has been updated in the production release
environment, we can use the bot user to override and merge the PR to
main.

## Testing
- Tested detection by deliberately specifying an old release and
reversing the `git diff` argument order to confirm tools change
detection
([dry-run](https://github.com/awslabs/aws-sdk-rust/actions/runs/19442709471/job/55629667389#step:5:69))

---

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

Author: ysaito1001

.github/workflows/release-checks.yaml

@@ -15,6 +15,10 @@ jobs:
     name: Check Release config.json
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/checkout@v4
+        with:
+          ref: main
+          path: main-branch
       - name: Check smithy-rs version exists
         run: |
           SMITHY_RS_VERSION=$(jq -r ".dependencies.smithyRsVersion" config.json)
@@ -23,11 +27,34 @@ jobs:
           cd smithy-rs
           git init
           git remote add origin https://github.com/smithy-lang/smithy-rs.git
-          
+
           if git fetch origin $SMITHY_RS_VERSION; then
             echo "smithy-rs@$SMITHY_RS_VERSION found"
           else
             echo "::error::no smithy-rs@$SMITHY_RS_VERSION found"
             exit 1
           fi
 
+      - name: Check for tools changes in smithy-rs
+        run: |
+          CURRENT_SMITHY_RS_VERSION=$(jq -r ".dependencies.smithyRsVersion" config.json)
+          echo "Current branch smithyRsVersion: $CURRENT_SMITHY_RS_VERSION"
+
+          MAIN_SMITHY_RS_VERSION=$(jq -r ".dependencies.smithyRsVersion" main-branch/config.json)
+          echo "Main branch smithyRsVersion: $MAIN_SMITHY_RS_VERSION"
+
+          cd smithy-rs
+          git fetch origin --tags -q
+
+          # Check for tools changes between main branch version and current version
+          CHANGED_FILES=$(git diff --name-only $MAIN_SMITHY_RS_VERSION..$CURRENT_SMITHY_RS_VERSION)
+
+          if echo "$CHANGED_FILES" | grep -q "^tools/"; then
+            echo "::error::Tools changes detected between $MAIN_SMITHY_RS_VERSION and $CURRENT_SMITHY_RS_VERSION"
+            echo "$CHANGED_FILES" | grep "^tools/"
+            echo "::error::Complete an internal release before updating config.json. After completion, use the bot user to force-merge this PR to main."
+            exit 1
+          else
+            echo "No tools changes detected - check passed"
+          fi
+