ci: pin npm 11.15.0 for stage publish command (#235)

The npm stage publish command requires npm 11.15.0+. Node 24's bundled
npm is 11.13.0, which doesn't have the command. The install step
already uses the npx -y -p npm@11.15.0 pattern; apply the same to the
publish step.

Author: Nelson-PROIA

.github/workflows/sdk_publish_mistralai_azure_sdk.yaml

@@ -51,4 +51,4 @@ jobs:
           fi
 
           echo "Staging under dist-tag: $TAG"
-          npm stage publish --tag "$TAG" --access public --provenance
+          npx -y -p npm@11.15.0 npm stage publish --tag "$TAG" --access public --provenance

.github/workflows/sdk_publish_mistralai_gcp_sdk.yaml

@@ -51,4 +51,4 @@ jobs:
           fi
 
           echo "Staging under dist-tag: $TAG"
-          npm stage publish --tag "$TAG" --access public --provenance
+          npx -y -p npm@11.15.0 npm stage publish --tag "$TAG" --access public --provenance

.github/workflows/sdk_publish_mistralai_sdk.yaml

@@ -61,4 +61,4 @@ jobs:
           fi
 
           echo "Staging under dist-tag: $TAG"
-          npm stage publish --tag "$TAG" --access public --provenance
+          npx -y -p npm@11.15.0 npm stage publish --tag "$TAG" --access public --provenance