Fix multi-endpoint Vision model menu and test connection

[chadpalmer]

Summary

• Fixes the Admin Settings Vision Model test button for multi-endpoint model deployments.
• Preserves endpoint/model metadata on Vision Model dropdown options while keeping the saved value compatible with existing deployment-name settings.
• Resolves the selected model endpoint server-side before running the Vision test, so models on non-legacy endpoints are tested against their configured endpoint.
• Removes duplicate _test_multimodal_vision_connection backend logic.
• Preserves GPT-5/o-series max_completion_tokens handling.
• Adds functional regression coverage and fix documentation.

Validation

• git diff --check upstream/Development
• python -m py_compile application/single_app/config.py application/single_app/route_backend_settings.py functional_tests/test_multimodal_vision_multi_endpoint_connection.py
• python functional_tests/test_multimodal_vision_multi_endpoint_connection.py
• python scripts/check_swagger_routes.py application/single_app/route_backend_settings.py
• python scripts/check_xss_sinks.py --base-sha upstream/Development --head-sha HEAD application/single_app/route_backend_settings.py application/single_app/static/js/admin/admin_settings.js application/single_app/templates/admin_settings.html
• python scripts/check_broken_access_control.py --base-sha upstream/Development --head-sha HEAD application/single_app/route_backend_settings.py
• python functional_tests/route_tests/test_route_blueprint_policy_inventory.py
• python functional_tests/route_tests/test_route_unauthenticated_policy_contract.py
• python functional_tests/route_tests/test_route_policy_test_coverage.py

Documentation

• Updated docs/explanation/release_notes.md under v0.250.008.
• Added docs/explanation/fixes/MULTI_ENDPOINT_VISION_TEST_CONNECTION_FIX.md.

Notes

• Branch is up to date with upstream/Development.
• Branch is pushed to origin/update-vision-model-section.
• No secret-bearing files or local generated artifacts are included.
• Browser UI tests were not run because no authenticated local browser app session was available during PR preparation.

[chadpalmer]

@Bionic711 @paullizer Here's another update for the admin settings menu that allows Vision Model multi-endpoint model deployments. It also fixes the Test Vision Model button as well. I ran the PR prep prompt, and everything passed. Please let me know if you need anything else from me.