ENG-3697: Improve eval sample upload progress

[d42me]

Adds eval sample upload progress reporting and reuses HTTP clients across batch uploads for faster prime eval push.

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Note

Medium Risk
Touches the eval sample upload path (sync+async) and changes concurrency/network behavior by reusing a single httpx client across batches; regressions would impact reliability/throughput of push_samples and prime eval push.

Overview
Adds optional progress_callback support to EvalsClient.push_samples and AsyncEvalsClient.push_samples, reporting skipped samples and per-batch upload counts.

Refactors sample uploads to reuse a single httpx.Client / httpx.AsyncClient per push (shared headers via _samples_upload_headers) instead of creating a new client per batch, and updates prime eval push to show a Rich progress bar when running in a terminal.

Extends tests to validate progress reporting, auth header propagation, and that only one HTTP client instance is created during batch uploads.

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