Fix duplicate slash URL handling for 25.9

[benflexcompute]

Summary

• Normalize Flow360 environment URL construction so leading path slashes do not create duplicate slash API paths.
• Remove known leading slash folder API paths.
• Constrain Requests to the 2.32 patch line.
• Bump the client version to v25.9.9 / 25.9.9 for release.
• Add focused URL normalization tests.

Verification

• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=/tmp/Flow360-25.9 /disk2/ben/Flow360/.venv/bin/python -m pytest tests/test_environment.py tests/test_current_flow360_version.py tests/simulation/test_updater.py -q -p no:cacheprovider
• Forced local repro with requests==2.34.2: prod and preprod folder requests used single-slash URLs and RestApi.get succeeded.

Note: poetry check --lock still fails on the existing unrelated pyswarms docs extra issue, but no longer reports stale lock metadata.

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Note

Low Risk
Targeted URL joining and path string tweaks with regression tests; no auth or data-model changes.

Overview
This release fixes API URL construction so folder and item calls no longer produce paths with double slashes when environment base URLs end with / and REST paths start with /.

EnvironmentConfig now joins base URLs and paths by trimming extra slashes on both sides (get_real_url, get_portal_real_url, get_web_real_url). Folder code uses v2/items and v2/folders without a leading slash on RestApi. requests is capped to the 2.32.x line, and the client is bumped to 25.9.9 with new tests for slash normalization and RestApi paths.

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