Prepare v0.2.3 release

[gabisonia]

• Fixed a cross-backend inconsistency in filter field handling:
  - Added shared field normalization (vectordata/filter_normalize.go) so Postgres and MSSQL treat whitespace/path validation the same.

  • Improved MSSQL performance path for filtering/search:
    • Added SQL filter pushdown compiler (stores/mssql/filter_pushdown.go).
    • Count now executes SQL-side filtering first, with safe fallback to in-memory eval if pushdown is unsupported.
    • SearchByVector now uses SQL-side distance/ranking plan (stores/mssql/search_sql.go) with fallback for unsupported filter shapes.

• Fixed MSSQL metadata-filter regression:
  - Switched metadata scalar predicates to JSON_VALUE/JSON_PATH_EXISTS semantics for reliable matches.

• Made EnsureIndexes behavior explicit on MSSQL:
  - Returns nil only when no index options are provided.
  - Returns a clear unsupported error when vector/metadata index options are requested.

• Added/updated tests to lock behavior:
  - vectordata/filter_normalize_test.go
  - vectordata/filter_sql_test.go
  - stores/mssql/filter_pushdown_test.go
  - stores/mssql/filter_eval_test.go
  - stores/mssql/collection_test.go