fix: Support Lineage Tracking for Top-Level and Nested Enums

[luthfifahlevi]

Overview
This PR fixes a bug in the Stencil Lineage API where requesting lineage for an Enum FQN (e.g., gotocompany.events.DataTypes.Enum) incorrectly returned an empty result (downstream_count: 0).

The root cause was a blind spot in the AST root lookup engine. The findRootFQNs and collectFQNs functions were exclusively iterating over Message types (fd.GetMessageType()), meaning Enum definitions were never registered as valid vertices in the lineage graph. Consequently, the traversal skipped them entirely.

Key Changes

• Top-Level Enum Support: Updated findRootFQNs to iterate over and index fd.GetEnumType(), ensuring file-level enums are registered in the graph lookup map.
• Nested Enum Support: Updated the recursive collectFQNs function to iterate over and index msg.GetEnumType(), allowing enums nested inside parent messages to be accurately targeted.
• Test Coverage: Added a dedicated descriptor builder (buildEnumDescriptorSetBytes) and subtest to verify that tracing downstream lineage starting directly from an Enum successfully identifies the parent messages and schemas that consume it.

Impact
This fix unlocks accurate downstream impact analysis for Enum modifications.