PYTHON-6084 Rework Evergreen Python interpreter selection for uv

[blink1073]

Rework how Evergreen and GitHub Actions select the Python interpreter and pin the uv binary. Task config now sets UV_PYTHON directly (a version like 3.10/3.14t, an implementation like pypy3.11, or an absolute path) so the interpreter lookup happens on the host. The uv binary version is pinned once in pyproject.toml via [tool.uv] required-version.

• Task config sets UV_PYTHON directly; removed TOOLCHAIN_VERSION everywhere and regenerated the Evergreen task and variant configs.
• setup-uv-python.sh resolves the interpreter on the host, setting UV_PYTHON_SEARCH_PATH and UV_PYTHON_PREFERENCE against a matching toolchain Python, and defaults to CPython 3.10 when a task does not pin one.
• uv is pinned via [tool.uv] required-version in pyproject.toml (==0.12.12) as a single source: uv enforces it locally, astral-sh/setup-uv reads it on GitHub, and Evergreen reads it too. Bumped manually.
• On CI, PYMONGO_BIN_DIR is ${TMPDIR:-/tmp}/pymongo-bin so the pinned uv stays isolated from the drivers-tools bin dir, and it is placed before that dir on PATH.
• just install installs the pinned uv into $HOME/.local/bin, and the justfile puts that directory first on PATH as a backstop.

PYTHON-6084

Changes in this PR

Test Plan

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?