ci(sdk): make GitHub Release creation idempotent

Address Copilot review: previously a successful run would create the
GitHub Release, and any retry (push the same tag again) would then
fail at this step even though the cargo publish wrapper handles
"already exists". Skip the create when a release for this tag is
already there, matching the cargo publish wrapper's behavior.

Author: kvinwang

.github/workflows/rust-sdk-release.yml

@@ -89,6 +89,10 @@ jobs:
           GH_TOKEN: ${{ github.token }}
           VERSION: ${{ steps.ver.outputs.version }}
         run: |
+          if gh release view "$GITHUB_REF_NAME" --repo "$GITHUB_REPOSITORY" >/dev/null 2>&1; then
+            echo "::notice::GitHub Release $GITHUB_REF_NAME already exists; skipping"
+            exit 0
+          fi
           gh release create "$GITHUB_REF_NAME" \
             --title "dstack-sdk $VERSION" \
             --notes "Published to crates.io: [dstack-sdk@$VERSION](https://crates.io/crates/dstack-sdk/$VERSION), [dstack-sdk-types@$VERSION](https://crates.io/crates/dstack-sdk-types/$VERSION)" \