Migrate dependency management from Pipenv to uv

[letitz]

This PR completes the migration of ClusterFuzz's Python dependency management system from Pipenv to uv. pipx is used to install uv in developer and CI/bot environments.

Architecture & Dependency Management

• Centralized pyproject.toml and uv.lock: Consolidated dependencies from root and App Engine Pipfiles into a single root pyproject.toml with [project.dependencies], [dependency-groups], and [project.optional-dependencies] (for vendored core, appengine, and platform-specific extras).
• Public PyPI Configuration: Configured [[tool.uv.index]] with PyPI (https://pypi.org/simple) as the default index to ensure lockfile references and wheels point to public PyPI rather than internal mirrors.
• Deterministic Tooling: Pinned pipx==1.10.0 and uv==0.12.3 across all setup scripts, container images, and CI configurations for setup determinism.
• Compatibility & Standards:
  • Maintained package versions matching the original Pipfiles.
  • Added setuptools==80.9.0 for pkg_resources API compatibility with legacy packages.
  • Pinned isort~=5.13.2 and pylint~=2.17.7 in dev dependencies for consistent linting across local and CI environments.
  • Removed deprecated Pipfile, Pipfile.lock, src/Pipfile, src/Pipfile.lock, and redundant generated requirements.txt files.

Butler & Build Tooling

• Vendoring with uv: Updated _install_third_party and _install_platform_pip in src/local/butler/common.py to use uv export --extra directly, eliminating intermediate requirements.txt generation.
• App Engine Unit Testing: Updated src/local/butler/py_unittest.py to preserve src/third_party in sys.path while adding src/appengine/third_party, ensuring both App Engine and core libraries are cleanly importable during App Engine unit tests.
• Environment Guards: Updated virtual environment guards in src/local/butler/guard.py and CLI runner in cli/casp/src/casp/utils/container.py to check for .venv and support uv run.

Setup Scripts, Bots & Container Images

• Robust Path & Fallback Handling: Dynamically query $(pipx environment --value PIPX_BIN_DIR 2>/dev/null || echo "$HOME/.local/bin") to ensure uv is on PATH across all setup scripts (local/install_python_deps_linux.bash, local/install_deps_macos.bash, local/install_python_from_source.bash) and bot startup scripts (configs/test/bot/setup/*.bash).
• Debian/Ubuntu Packaging: Added python3-venv to install_deps_linux.bash so Python virtual environments can be built without relying on global pip.
• Windows GCE Setup: Updated configs/test/gce/windows-init.ps1 to install uv via pipx and append the pipx binary directory to $env:Path.
• Base Docker Images: Updated base Dockerfiles (docker/base/Dockerfile, docker/base/ubuntu-20-04.Dockerfile, docker/base/ubuntu-24-04.Dockerfile) to install uv via pipx and symlink uv into /usr/local/bin.

CI Pipelines

• Deduplicated CI Execution: Extracted the container-based CI test pipeline into local/tests/ci_tests_inner.bash, shared between Cloud Build (local/tests/cloudbuild_ci.bash / deploy/cloudbuild.yaml) and GitHub Actions (local/tests/ci_tests.bash / .github/workflows/build.yaml).
• Fresh Checkout Integration Test: Added local/tests/install_deps_test_linux.bash to verify end-to-end repository installation and testing from a clean checkout.

Documentation

• Updated AGENTS.md, local/README.md, .devcontainer/devcontainer.json, and developer documentation to reflect uv workflows (uv run butler.py ...).

TAG=agy
CONV=1906a716-4a06-47e6-b2fb-fc2038c78ef6

[letitz]

First pass at self-review.