Exit with zero when uv tool install request is already satisfied

[billy-doyle]

Summary

uv tool install X fails for tools already installed and exits with code 1. Allow already installed tools to exit with code 0.

Test Plan

[zanieb]

In theory, there will be a test snapshot that will fail because of this change in tool_install.rs

[billy-doyle]

@zanieb modified the tests

tested locally with cargo test tool_install. Are these "success: true exit_code: 0 ----- stdout ----- ..." autogenerated? I modified by hand but i did see in CONTRIBUTING.md cargo dev generate-json-schema although it did not fix anything there

[zanieb]

@billy-doyle usually you'd do cargo install cargo-insta and cargo insta test -- <test-name>. I'm not sure why we don't cover that in the contributing guide! We ought to add it.