fix(update): check version exists before confirming uninstall

[euxaristia]

Summary

Prevents spacetime version uninstall <ver> from showing a confirmation prompt when the version isn't installed, which previously resulted in a cryptic "No such file or directory (os error 2)" error after the user confirmed.

Changes

Bug fix

• Check if the version directory exists before showing the y/N prompt
• Return a clear error message: v{version} is not installed

Tests (4 unit tests)

• test_uninstall_nonexistent_version_errors_before_prompt — confirms error fires before prompt for missing versions
• test_uninstall_current_version_errors — confirms you can't uninstall the active version
• test_uninstall_current_keyword_errors — confirms the literal string "current" is rejected
• test_uninstall_existing_version_with_yes — confirms normal uninstall flow works

Verification

cargo check -p spacetimedb-update
cargo clippy -p spacetimedb-update -- -D warnings
cargo test -p spacetimedb-update uninstall

Reproduction

Before this fix:

$ spacetime version uninstall 2.0.3
Uninstall v2.0.3? yes
Error: No such file or directory (os error 2)

After this fix:

$ spacetime version uninstall 2.0.3
Error: v2.0.3 is not installed

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[bfops]

Hi @euxaristia, thanks for opening a PR. Is there a reason for the changes in crates/bindings-cpp-ffi?

[euxaristia]

Sorry that was mistakenly included. I think I made a separate PR for those changes already. I'll remove the bindings noise from this PR soon.