feat(examples): backport download_benchmark_assets to 25.8

[marcvivesmassana]

Summary

• Backports the download_benchmark_assets helper from https://github.com/flexcompute/compute/pull/6942 into the release-candidate/25.8 client line.
• Adds flow360/examples/benchmark_assets.py (pure urllib + json, no new dependencies) and exports download_benchmark_assets from flow360.examples.
• Lets the Flow360 benchmark reproducer notebooks fetch a case's public root asset and reference data with no credentials or private project id, on the 25.8 client family the cases are validated against.

Context

The helper currently ships only on the 25.10 client (https://github.com/flexcompute/compute/pull/6942). The benchmark reproducer notebooks pin older client families (25.8 / 25.9), so the helper has to be backported to those lines for the notebooks to run as-is.

Test Plan

• from flow360.examples import download_benchmark_assets imports cleanly.
• download_benchmark_assets("NACA_0012", "root_assets") returns the local mesh path from the public benchmark bucket.

🤖 Generated with Claude Code

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Note

Low Risk
Additive examples helper with path traversal guards; no auth, billing, or core solver changes.

Overview
Backports download_benchmark_assets onto the 25.8 client so benchmark reproducer notebooks can pull published case files without credentials or hard-coded URLs.

Adds flow360/examples/benchmark_assets.py, which fetches from the public simcloud-public-1 benchmark bucket via urllib and json only. Callers pass case_id and kind (root_assets for geometry/mesh snapshots, or folders like ref_data); multi-project root assets require project_id. Downloads land under to_folder, with input data recreated under to_folder/<kind>/…. Manifest paths are validated so writes cannot escape the destination directory.

Exports the helper from flow360.examples (__init__.py import and __all__).

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