localenv: add venv-folder and per-file pyproject seams to setup-local

[rugpanov]

Stack (merge in this order):

1. localenv: add venv-folder and per-file pyproject seams to setup-local #7017 localenv: add venv-folder and per-file pyproject seams to setup-local (see diff) <- this PR
2. Support uv workspace members in environments setup-local #7018 Support uv workspace members in environments setup-local (see diff)
3. Honor UV_PROJECT_ENVIRONMENT in environments setup-local #7026 Honor UV_PROJECT_ENVIRONMENT in environments setup-local (see diff)

Changes

This PR is a refactor in libs/localenv. It does not change behavior. It adds two seams that #7018 uses for uv workspace members:

• PostProvision and Validate take the project folder and the venv folder. uv and the validate probe still run in the project folder. Preflight sets the venv folder; for every project it is still .venv.
• The pyproject.toml backup, write, and dry-run diff are per-file helpers (writePyproject, pyprojectDiff, and backup helpers that take a folder). This lets Support uv workspace members in environments setup-local #7018 write a second file, the workspace root pyproject.toml.

Why

In a uv workspace member, uv sync puts the venv at the workspace root, and uv reads the constraint pins only from the root pyproject.toml (databricks/databricks-vscode#2157). The pipeline used one fixed venv path and one pyproject.toml. This PR makes both locations parameters, so #7018 can change them for a member without a mixed refactor-and-feature diff.

Tests

• Unit tests: the helpers write the backup next to the file, a no-op merge writes nothing, and uv and the probe run in the project folder.
• All existing unit and acceptance tests pass with no golden change. This shows that the output does not change.

This pull request and its description were written by Isaac.

[eng-dev-ecosystem-bot]

Integration test report

Commit: 29539ed

Run: 38245862518

	Env	✅​pass	🙈​skip	Time
✅​	aws linux-2core-8gb	276	16	6:58
✅​	aws-windows-latest-4core-16gb	278	14	3:23
✅​	azure linux-2core-8gb	275	16	7:06
✅​	azure-windows-latest-4core-16gb	277	14	2:56
✅​	gcp linux-2core-8gb	276	16	6:56
✅​	gcp-windows-latest-4core-16gb	278	14	3:00

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
3:57	aws linux-2core-8gb	TestAccept
3:56	azure linux-2core-8gb	TestAccept
3:45	gcp linux-2core-8gb	TestAccept