Name the conflicting ref in ExistingRefError

[asadbek0512]

Fixes #11434

ExistingRefError.Error() returned a bare already exists. A user seeing it learns neither which ref was taken nor what kind it was. The TODO(elianddb) on it said the ref could only be named once tags and workspaces used their own errors, since they surfaced this message verbatim.

Changes

• actions.ErrTagExists / actions.ErrWorkspaceExists, mirroring the existing ErrBranchExists wording.
• CreateTagOnDB and CreateWorkspaceOnDB return those instead of &doltdb.ExistingRefError{...}.
• ExistingRefError.Error() now returns ref 'refs/heads/br' already exists; the TODO is gone.

Why tags and workspaces return their error at the source

BranchExistsError exists because failOnCaseConflict raises ExistingRefError deep inside branch creation, where the ref that actually collides differs from the one the caller asked for; the caller has to unwrap to learn the real name. Tag and workspace creation instead detect the duplicate with an explicit ddb.HasRef check that already has the name in scope, and neither path runs failOnCaseConflict. An unwrapper for them would only re-derive a name the raising function held, so they return the typed error directly. CreateWorkspace currently has no callers, which also makes an unwrapper dead code.

That leaves ExistingRefError raised only on branch paths, whose callers already wrap it. Its own message is the fallback for a caller that does not, so it uses Ref.String() rather than GetPath(): a name can exist as more than one ref type, and an unwrapped error should not be ambiguous about which one it means.

Compatibility

No test asserted the bare string. branch_case_test.go matches on the error type and Ref.GetPath(), both unchanged. No enginetest or bats case asserts a tag- or workspace-exists message; the bats suites that grep for already exists cover tables, foreign keys, and files, and the new messages still contain that substring.

Testing

go test passes for ./libraries/doltcore/doltdb/..., ./libraries/doltcore/env/..., and -run 'TestDoltTag|TestDoltBranch|TestDoltCheckout' in ./libraries/doltcore/sqle/enginetest/. go vet is clean on the touched packages. Added TestExistingRefErrorNamesTheRef to pin the message across branch, tag, and workspace refs.