ATLAS-5159:- Export APIs: Exporting parent type doesn't export child type

[UmeshPatil-1]

ATLAS-5159
What changes were proposed in this pull request?

Background:
Exporting entities of a parent type (e.g., Referenceable) does not export entities of its child type (e.g., hive_table).
When attempting to export using the parent type with unique attributes, no child entities were returned, leading to incomplete exports.

Root Cause:

The previous implementation of StartEntityFetchByExportRequest did not consistently account for the type hierarchy. Specifically, when fetching by uniqueAttributes/matchType, the logic did not automatically include subtypes, meaning only entities explicitly labeled with the parent type were discovered.

Changes Proposed:

Hierarchical Type Lookup: Updated getEntitiesForMatchTypeUsingUniqueAttributes to identify the AtlasEntityType and automatically include all its subtypes (entityType.getAllSubTypes()) in the search criteria.

Gremlin Binding Expansion: Refactored getBindingsForTypeName to handle comma-separated type names and recursively resolve all subtypes for each provided type. This ensures that a query for a parent type translates into a Gremlin search across the entire inheritance tree.

Enhanced Match Logic:
Improved getEntitiesForMatchTypeType to utilize the expanded type list.

Impact:

Exporting parent types like Referenceable now correctly includes all child entities.
Maintains consistent behavior for all fetch types (FULL, INCREMENTAL, CONNECTED).
Minimal impact on performance; no additional schema changes are required.

How was this patch tested?

Maven Build:- Build Successfull.
Manual testing of export requests with all three fetch types: FULL, INCREMENTAL, CONNECTED.
Verified entity GUIDs are correctly returned for parent types and child entities,Concrete types with unique attributes (hive_table)
Confirmed behavior for concrete types, abstract types, and invalid/misspelled types.
Logs verified query execution, fetch type handling, and error reporting.