ci: add manual pre-release workflow for v3 branch

.github/workflows/v3_pypi_release.yml

@@ -0,0 +1,43 @@
+name: Manual pre-release from v3 on PyPI
+
+on:
+  workflow_dispatch:
+
+env:
+  HATCH_VERSION: "1.16.5"
+
+jobs:
+  v3-release:
+    runs-on: ubuntu-latest
+    environment: pypi
+    permissions:
+      id-token: write
+    steps:
+      - name: Checkout v3
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+        with:
+          ref: v3
+          fetch-depth: 1
+
+      # Reads VERSION.txt, strips any -rcN suffix, and appends .devYYYYMMDDHHMMSS
+      # (e.g. 3.0.0.dev20260519000000) so each run gets a unique, PEP 440–valid pre-release version.
+      - name: Set pre-release version
+        id: set-version
+        run: |
+          BASE_VERSION=$(sed 's/-rc[0-9]*$//' VERSION.txt)
+          TIMESTAMP=$(date +%Y%m%d%H%M%S)
+          PRERELEASE_VERSION="${BASE_VERSION}.dev${TIMESTAMP}"
+          echo "version=${PRERELEASE_VERSION}" >> "$GITHUB_OUTPUT"
+          echo "${PRERELEASE_VERSION}" > VERSION.txt
+          echo "Building haystack-ai version: ${PRERELEASE_VERSION}"
+
+      - name: Install Hatch
+        run: |
+          python -m pip install --upgrade pip
+          pip install hatch==${{ env.HATCH_VERSION }} --uploaded-prior-to=P1D
+
+      - name: Build Haystack
+        run: hatch build
+
+      - name: Publish to PyPI
+        uses: pypa/gh-action-pypi-publish@cef221092ed1bacb1cc03d23a2d87d1d172e277b # v1.14.0