Add REST API for parallel test balancing

What does this MR do and why?

This MR adds the REST API layer for parallel test balancing, building on the Redis-backed queue introduced in the parent MR. It exposes two endpoints under /job/test_balancing that let a running parallel CI/CD job claim duration-budgeted batches of test splits from a job group's shared pending pool, so work is balanced across nodes.

API summary

Endpoint	Method	Description
/job/test_balancing/initialize	POST	Seeds the job group's shared pending pool with the caller's static test split, then claims a first batch. On a job retry or crash recovery, returns the node's previously claimed set unchanged.
/job/test_balancing/request	POST	Atomically claims a duration-budgeted batch of pending test splits for the calling node, slowest first. An empty result means the queue is drained.

The new paths the pattern of /job/allowed_agents which is also an endpoint authenticated by a CI job token and fetches information related to the current job.

Feature availability

• Gated behind the parallel_test_balancing feature flag (beta, disabled by default).
• Requires the ci_parallel_test_balancing licensed feature (Ultimate).
• Endpoints are currently hidden and marked as experiment.

Related to #607450 (closed)