feat(namespace-dir): implement table tag operations

[XuQianJin-Stars]

Summary

Implement the 5 table tag trait methods on DirectoryNamespace by delegating to the native lance Dataset::tags() interface, so that tag management works end-to-end against any directory-backed namespace (local FS / object store) without requiring a remote catalog.

Closes #6885.

What's Changed

Implemented the following TableTag trait methods on DirectoryNamespace (rust/lance-namespace-impls/src/dir.rs):

• list_table_tags — list all tags of a table, with their referenced version and an optional message.
• get_table_tag_version — resolve a tag name to its dataset version.
• create_table_tag — create a new tag pointing to a given version; rejects duplicates.
• delete_table_tag — delete an existing tag; rejects unknown tags.
• update_table_tag — move an existing tag to a new version; rejects unknown tags / versions.

All methods open the target dataset via the existing directory-resolution logic and then call into Dataset::tags() (create / update / delete / list / get_version).

Error Mapping

A small helper map_tag_error translates lance-core ref errors into the proper NamespaceError variants used by the namespace API:

lance-core error	NamespaceError
RefNotFound	TableTagNotFound
RefConflict	TableTagAlreadyExists
InvalidRef	InvalidInput
VersionNotFound	TableVersionNotFound

Additionally, requests are validated up-front:

• Empty tag names → InvalidInput.
• Non-positive versions (<= 0) on create / update → InvalidInput.
• Missing tables continue to surface as TableNotFound via the existing dataset-open path.

Tests

Added 10 unit tests in dir.rs covering:

1. create + list happy path (tag visible with correct version & optional message).
2. create conflict on duplicate tag → TableTagAlreadyExists.
3. get_table_tag_version happy path.
4. get_table_tag_version on unknown tag → TableTagNotFound.
5. update_table_tag happy path (version moves forward).
6. update_table_tag on unknown tag → TableTagNotFound.
7. delete_table_tag happy path (subsequent get fails).
8. delete_table_tag on unknown tag → TableTagNotFound.
9. Invalid input: empty tag name and non-positive version → InvalidInput.
10. Operations on a non-existent table → TableNotFound.

Scope / Non-Goals

• Only DirectoryNamespace is touched; other namespace implementations are unchanged.
• No changes to public trait signatures, the namespace REST schema, or the on-disk tag format — this PR purely wires the existing Dataset::tags() API through the directory namespace.

[claude]

Claude Code Review

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[codecov]

Codecov Report

❌ Patch coverage is 89.69957% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-namespace-impls/src/dir.rs	89.69%	24 Missing ⚠️

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