chore(ci): migrate Python tooling to uv and standardize workflow execution (#7386)

# Description of Changes

This PR modernizes the project's Python tooling across GitHub Actions by
migrating CI workflows from pip-based dependency management to `uv` and
aligning Python execution with the engine project's managed environment.

### What was changed

- Replaced `actions/setup-python` and ad-hoc `pip install` steps with
`astral-sh/setup-uv` across CI workflows.
- Configured shared `uv` dependency caching using
`engine/pyproject.toml` and `engine/uv.lock`.
- Updated Python script execution to use `uv run --project engine
--locked` for a consistent runtime environment.
- Replaced package installation steps with `uv sync` for the required
dependency groups (e.g. `tools` and `cucumber`).
- Added Docker image build validation for both production and
development AI engine images.
- Updated workflow cache configuration and Docker build context where
required.
- Removed obsolete Python requirements files that are no longer needed
after the migration.
- Applied minor Python code modernizations, including import cleanup,
modern built-in generic type annotations (`list[...]`, `tuple[...]`,
`float | None`), and small style improvements.
- Removed unnecessary Python formatter/linter extensions from the
development container configuration.

### Why the change was made

- Standardize Python dependency management across the repository.
- Reduce duplicated dependency installation logic in CI.
- Improve workflow performance through shared dependency caching.
- Ensure all Python utilities execute against the same locked dependency
set managed by the engine project.
- Simplify long-term maintenance by eliminating legacy requirements
files and pip-specific workflow steps.


---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

---------

Signed-off-by: Carsten Drewes <c.drewes@stud.uni-hannover.de>
Co-authored-by: albanobattistella <34811668+albanobattistella@users.noreply.github.com>
Co-authored-by: kastenherri <116314318+kastenherri@users.noreply.github.com>
Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: James Brunton <jbrunton96@gmail.com>

testing/compose/run-multinode-regression.sh

@@ -25,12 +25,8 @@ elif [ "$SEED" = 1 ]; then
 fi
 
 echo "==> Checking Python + behave..."
-PY="${PYTHON:-python}"
-command -v "$PY" >/dev/null || PY=python3
-if ! "$PY" -c "import behave" 2>/dev/null; then
-  echo "    installing test deps..."
-  "$PY" -m pip install -q -r "$CUKE_DIR/requirements.txt" || {
-    echo "    could not install behave; install $CUKE_DIR/requirements.txt manually"; exit 1; }
+if ! uv run --project ../../engine --locked --group cucumber python -c "import behave" 2>/dev/null; then
+  echo "    could not load the central uv cucumber environment"; exit 1
 fi
 
 REPORT_DIR="$(pwd)/multinode/regression-report"
@@ -39,7 +35,7 @@ mkdir -p "$REPORT_DIR"
 run_behave() { # $1=tags  $2=label
   echo "==> behave features/multinode --tags='$1'  ($2)"
   # behave.ini excludes features/multinode by default; -e here overrides that while still excluding the licence-gated enterprise suite.
-  ( cd "$CUKE_DIR" && "$PY" -m behave features/multinode -e "features/enterprise" \
+  ( cd "$CUKE_DIR" && uv run --project ../../engine --locked --group cucumber python -m behave features/multinode -e "features/enterprise" \
       --tags="$1" --no-capture --format plain --format html --outfile "$REPORT_DIR/$2.html" )
   return $?
 }