Refactor datastore internals and tighten query option safety

[squeakycheese75]

Motivation

• Improve cohesion by splitting datastore logic into files grouped by concern and make client behavior clearer.
• Ensure consistent, safe query behavior where empty result sets return an empty slice rather than an error.
• Harden query option validation so callers cannot rely on implicit inference of allowed columns from user input.
• Improve error messages and use %w wrapping for better error propagation.

Description

• Split internal/datastore into focused files: client.go, query_options.go, postgres_client.go, sqlite_client.go, sqlite_storage.go, plus existing factory.go, sqlite_storage_local.go, and sqlite_storage_s3.go.
• Renamed SqliteDataStoreClient to SQLiteDataStoreClient and kept constructor APIs NewSQLiteDataStoreClient / NewSQLiteDataStoreClientWithStorage intact.
• Changed Find and Query in both SQLite and Postgres clients to return []map[string]interface{}{} with nil error when no rows are found, and improved error messages using %w wrapping where appropriate.
• Tightened ApplyQueryOptionsWithSafety so it no longer infers AllowedCols from Filters/OrderBy when AllowedCols is empty and now validates/returns an error for invalid filter or order columns.
• Cleaned up factory error messages to use %w wrapping and corrected a Postgres unmarshal error label.
• Added internal/datastore/client_query_test.go to cover empty-result handling and invalid query options.

Testing

• Added unit tests in internal/datastore/client_query_test.go covering empty results for Find/Query and invalid QueryOptions, and they passed when running the package tests.
• Ran go test ./internal/datastore and it completed successfully.
• Ran go test ./... and the full test suite completed successfully (no failures reported).

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Codex Task