Stage Windows self-updates before replacing uv.exe - #20855

### Summary

Fix Windows self-updates so an interrupted or failed installation does not leave uv unusable.

Instead of renaming the current uv.exe before running the installer, the updater now stages the new release in a temporary directory. The existing executable remains in place until installation completes successfully, after which uv replaces itself using the Windows-safe self-replacement mechanism.

Addresses #12142.

Changes

• Install the new release into a temporary directory before replacing the running executable.

• Keep the existing uv.exe in place while the external installer runs.

• Replace uv.exe from the staged copy only after installation succeeds.

• Copy companion binaries from the temporary installation into the existing executable directory.

• Run the temporary installer with:

  • an isolated configuration directory;
  • PATH modification disabled.

• Preserve the existing install receipt and shell configuration.

• Configure the updater child process to terminate when the parent task is interrupted.

• Remove the previous flow that renamed uv.exe to .previous.exe before installation.

The old flow could leave the canonical executable missing when an update was interrupted. With this change, failed or interrupted installations leave the existing uv.exe available.

Testing

Added a Windows regression test that:

1. Starts a self-update installer.
2. Interrupts it after installation begins.
3. Verifies that the original executable is still present.

The regression test failed against the previous implementation and passes with this change.

Validation completed locally:

cargo test -p uv --lib --features self-update -- --nocapture
# 43 passed

cargo fmt --all --check

cargo clippy -p uv --all-targets --all-features --locked -- -D warnings

Not run locally:

• Full workspace and integration test suite.
• A self-update using the real release installer.

These are best covered by CI.