fix: return an error when uninstalling a non-installed version

[suman2280]

Summary

solves #616

tenv <tool> uninstall <version> used to silently report success when the requested version was not installed locally. This change makes it return a clear <Tool> <version> is not installed error and a non-zero exit code, matching the documented expected behaviour.

Fixes the bug described for tf, tofu, tg (and tm) where running

tenv <tool> uninstall 0.0.0

reported Uninstallation of <Tool> 0.0.0 successful (directory … removed) even though the version does not exist.

What this PR fixes

uninstallSpecificVersion in versionmanager/manager.go relied on os.RemoveAll, which is a no-op on a non-existent path and returns nil. Callers therefore reported success even when nothing was installed for the requested version.

Implementation

versionmanager/manager.go

• New sentinel errVersionNotInstalled exported package-wide.
• uninstallSpecificVersion stats the target directory first; missing path -> display the user-facing message and return wrapped errVersionNotInstalled; otherwise os.RemoveAll and only report success when it actually removes something.
• Uninstall propagates the error in both branches: exact version and the constraint path (when SelectVersionsToUninstall yields an empty selection).
• UninstallMultiple and the multi-version loop in Uninstall collect per-version errors via errors.Join.

versionmanager/tenvlib/lib.go

• Tenv.Uninstall now delegates to manager.Uninstall(requestedVersion) instead of UninstallMultiple([]string{requestedVersion}), routing the single-version CLI path through the direct, non-interactive branch and ensuring the new error reaches callers.

Tests

versionmanager/uninstall_test.go (new):

• Uninstall_not_installed_exact_version -- original bug.
• UninstallMultiple_returns_error_for_not_installed.
• UninstallMultiple_returns_joined_errors_for_multiple_not_installed -- verifies errors.Join wrapping.
• Uninstall_installed_version_succeeds -- happy path regression.
• Uninstall_same_version_twice_returns_error -- idempotency.

Commits

This PR is split into two commits to keep history readable:

1. fix(uninstall): return an error when the target version is not installed -- manager logic + tests.
2. fix(tenvlib): route single-version Uninstall through VersionManager.Uninstall -- public-API routing.

Test plan

• go build ./...
• go vet ./...
• gofmt -l versionmanager/ clean
• go test ./versionmanager/... ./versionmanager/tenvlib/... -- all new subtests pass; no regressions in existing suites.

[suman2280]

@kvendingoldo Can you please review this?