Resolve issue-3264: improve performance and documentation of unclaimed_sv_rewards

[jose-velasco-ieu]

Description

This PR resolves #3264 by improving the performance, robustness, and documentation of the unclaimed_sv_rewards.py script.

Key changes

• Parallelized Scan processing by splitting the requested time range into
  independent time chunks.
• Added support for multiple Scan URLs, used in a round-robin fashion per
  chunk to increase throughput and provide failover.
• Implemented retry logic with automatic Scan server rotation.
• Tuned default concurrency-related parameters based on empirical testing.
• Improved CLI help messages for clarity and correctness.
• Added and updated RST documentation for the script.
• Updated integration tests and performed minor cleanup.

Concurrency model (high level)

The script divides the input time range into small chunks that can be
processed independently. Each chunk is handled asynchronously and uses a
rotated Scan URL. Results are merged sequentially to ensure deterministic
behavior regardless of concurrency level.

Pull Request Checklist

Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a hard-migration test is required (from the latest release), comment /hdm_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.

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