feat: uv support for Flask, Django and FastAPI extensions

[Thanhphan1147]

Summary

Adds uv build support to the Flask, Django and FastAPI framework extensions. When a project ships both uv.lock and pyproject.toml, the extension builds the dependencies part with the uv plugin (uv sync) instead of the python plugin, and injects the server dependency (gunicorn~=23.0 / uvicorn) via a post-build override-build step since the lockfile is the source of truth.

Behavior

• uv.lock + pyproject.toml → build with uv plugin.
• pyproject.toml only → fall back to the python (pip) plugin (unchanged).
• uv.lock without pyproject.toml → pack fails: "the plugin requires both uv.lock and pyproject.toml to be present".
• Django/FastAPI no longer require requirements.txt for uv projects.

Testing

• Unit tests for detection, part generation and validation across all three extensions.
• make lint passes; extension unit tests pass.
• Manual e2e on a Canonical k8s cluster: built a Flask uv rock, pushed to an in-cluster registry, ran it as a pod, and confirmed HTTP 200 served by gunicorn 23.0.0.

[Copilot]

Pull request overview

This PR adds uv-based build support to the Flask, Django, and FastAPI framework extensions so that projects with both uv.lock and pyproject.toml use the uv plugin for dependency installation, while preserving existing pip/python plugin behavior when uv.lock is absent.

Changes:

• Add uv project detection + validation helpers (uses_uv, validate_uv_lockfile) and wire them into Flask/Django/FastAPI extension validation/part generation.
• Switch dependency parts to use the uv plugin when appropriate, injecting gunicorn/uvicorn via override-build for locked (uv) projects.
• Add unit tests and documentation sections describing uv behavior for all three extensions.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/unit/extensions/test_python_utils.py	New unit tests for uv detection/validation helpers.
tests/unit/extensions/test_gunicorn.py	Adds Flask/Django uv-project tests and error-case coverage.
tests/unit/extensions/test_fastapi.py	Adds FastAPI uv-project tests and error-case coverage.
rockcraft/extensions/gunicorn.py	Uses uv plugin for deps when uv.lock+pyproject.toml are present; validates lockfile pairing.
rockcraft/extensions/fastapi.py	Uses uv plugin for deps when uv.lock+pyproject.toml are present; skips requirements.txt checks for uv projects.
rockcraft/extensions/_python_utils.py	Introduces uses_uv() and validate_uv_lockfile() helpers.
docs/reference/extensions/flask-framework.rst	Documents uv project behavior for Flask extension.
docs/reference/extensions/django-framework.rst	Documents uv project behavior for Django extension.
docs/reference/extensions/fastapi-framework.rst	Documents uv project behavior for FastAPI extension.

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