feat(controller): Add dependent target lookup

[prathshenoy]

Summary

What:

• Add a streamed API that returns targets transitively dependent on requested targets at a specific revision.

Why:

• Stovepipe needs to decide which services require validation for a change. It can identify changed build targets, but it cannot currently ask Tango which other targets transitively depend on them. This endpoint provides that missing graph query so Stovepipe can expand changed targets into the set of potentially affected service targets before selecting validations.
• Without this API, Stovepipe would need to retrieve and traverse Tango’s target graph itself, duplicating graph-format handling and reverse-dependency logic outside the system that owns the graph. Keeping traversal in Tango lets Stovepipe consume a small, service-resolution-oriented result while Tango supports both its compact TGB representation and legacy graph storage.

Test Plan

• Add unit tests covering dependent-target traversal, request validation, and streamed response batching.

Revert Plan

• Revert this PR; no existing caller depends on the new endpoint yet, so removing the additive API requires no client migration or data cleanup.

Issues

• CODEM-535