Context

PyMongo's Evergreen tasks uses a confusing mix of approaches to finding the right Python, and also lacks the ability to install a pre-release Python (such as 3.15rc1) that is not already in the toolchain. This ticket blocks 
PYTHON-6077.

A later improvement tracked in DRIVERS-3564 will let us use ensure_uv.sh from drivers-evergreen-tools to find or install uv with an explicit version specifier. This ticket is not blocked by that work.

Definition of done

• Tasks request a Python through UV_PYTHON: a version (3.10), a variant (3.14t, pypy3.11), or an explicit interpreter path (for example the FIPS host's /usr/bin/python3.11). TOOLCHAIN_VERSION is removed from the task generator and the generated configs.
• On the host, a matching Python in the Python toolchain is used directly. When there is no matching one, uv supplies a Python. If uv still cannot provide the requested Python (for example it never shipped a build for that version), the task fails with a clear error.
• The default UV_PYTHON remains 3.10, but will be found in the Python toolchain if available.
• Behavior is validated on Linux, macOS arm64, Win64, Win32, the FIPS host, and a toolchain-less host (Amazon Linux).
• Evergreen task and variant names are unchanged, and regenerating the configs is deterministic.
• An evergreen patch build demonstrating that Python 3.15 is installed using the latest uv.

Pitfalls

• An empty search-path environment variable disables uv's PATH-based discovery entirely, so it must only be exported when it has a value.
• An explicit interpreter path must be used as-is and never routed through uv's install step.
• Generated task and variant names must stay byte-identical after the generator change.