Add integration tests for frontend path

[tartansandal]

Introduces an integration test for #5674.
This xfails on main but passes on #5698

Edit: also fixed an apparent bug in test_link_hover.py. There may be a deeper issue here.
Edit: removed xfail after merging main which now includes #5698.
Edit: added a test for 404s.

[codspeed-hq]

CodSpeed Performance Report

Merging #5712 will not alter performance

_{Comparing tartansandal:kjh/integration-tests-for-frontend-path (bbe8847) with main (5f3bf56)}

Summary

✅ 8 untouched

[greptile-apps]

Greptile Summary

This PR introduces integration testing for frontend path functionality, specifically addressing issue #5674 where on_load and on_mount handlers fail to redirect properly when Reflex applications are served from non-root frontend paths (e.g., '/prefix').

The changes include two main components:

1. Environment Variable Addition: A new REFLEX_FRONTEND_PATH environment variable is added to the EnvironmentVariables class in reflex/environment.py. This string variable defaults to an empty string and provides the configuration mechanism for specifying the base path under which the frontend is served.

2. Comprehensive Integration Test: A new test file tests/integration/tests_playwright/test_frontend_path.py creates a complete end-to-end test using Playwright. The test creates a Reflex app with redirect pages and validates that redirects work correctly both in normal conditions and when served from a non-root frontend path.

The integration test uses two fixtures - one for production mode and another that sets the REFLEX_FRONTEND_PATH to '/prefix'. It employs parametrized testing to verify both scenarios, with the prefix case marked as xfail since it demonstrates the known bug that should be resolved by the related PR #5698.

This change fits into the broader Reflex ecosystem by enabling proper deployment behind reverse proxies, a common pattern where multiple applications are served from different paths on the same domain. The environment variable provides a clean configuration interface that other parts of the codebase can reference to adjust their behavior for non-root deployments.

Confidence score: 4/5

• This PR introduces well-structured integration testing with minimal risk to existing functionality
• Score reflects solid test design and environment variable implementation, though the xfail test indicates known issues
• Pay close attention to the test file to ensure proper cleanup of environment variables and test isolation

_{2 files reviewed, 2 comments}

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[tartansandal]

Note I'm using simple environment module tweaks to simulate changing the environment. An alternative would be to modify AppHarness to take a config object. That would be a much larger change, but might be useful testing some configuration scenarios.