PYTHON-6091 Pin a consistent uv binary version in CI and locally

[blink1073]

Ensure a consistent uv binary is used in Evergreen CI, GitHub Actions, and local development, based on a single known version.

• Pins the uv binary version via [tool.uv] required-version in pyproject.toml, enforced locally and read by astral-sh/setup-uv.
• .evergreen/scripts/install-dependencies.sh discovers uv from the toolchain (falling back to PATH, then astral), installs the exact pinned version with uv tool install, and ensures just is present — all into a task-local $PYMONGO_BIN_DIR.
• On CI configure-env.sh puts PYMONGO_BIN_DIR in ${TMPDIR:-/tmp}/pymongo-bin (a space we control) ahead of the drivers-tools bin dir, and keeps it in the form bash searches on PATH on Windows.
• justfile puts the local bin dir first on PATH as a backstop.
• Removed the toolchain-symlink bootstrap; uv/uvx/just are discovered and installed instead.

PYTHON-6091

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• 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).

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