Improve test runner speed

[Archmonger]

Description

Test runner takes a long time to run, even on a high performance Windows machine. This is largely due to Playwright startup/shutdown being slow.

This PR does the following

• Changes to test suite to allow hatch (pytest) to run in parallel mode
• Changes to the build process to avoid extraneous rebuilds in parallel mode
• Fix bugs in source code which caused flaky tests (related to ReactJS double-mounting)
• Move some functions around
• Pin package versions within component_from_npm tests

On my local machine: Reduced from ~150 seconds to ~20 seconds.
On GitHub Actions: Reduced from ~15 minutes to ~5 minutes.

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[Copilot]

Pull request overview

This PR significantly improves test runner performance by enabling parallel test execution and addressing related infrastructure issues. The changes reduce test execution time from ~150 seconds to ~20 seconds locally and from ~15 minutes to ~6 minutes in CI.

Key improvements include:

• Session-scoped browser fixtures to avoid repeated Playwright startup/shutdown
• File locking mechanism to prevent race conditions during parallel module registration
• Dynamic port allocation for test servers
• React/Preact framework detection to handle double-mounting correctly
• Unique module names in tests to prevent conflicts in parallel execution

Reviewed changes

Copilot reviewed 44 out of 47 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/conftest.py	Converted fixtures to session scope, removed rebuild logic, simplified browser initialization
src/reactpy/testing/display.py	Added browser/backend externality tracking, improved page configuration, added visibility helper
src/reactpy/testing/backend.py	Implemented dynamic port allocation (port=0), added server startup validation loop
src/reactpy/testing/common.py	Removed clear_reactpy_web_modules_dir utility (no longer needed with locking)
src/reactpy/reactjs/utils.py	Added file locking for module registration, improved atomic file copy operations
src/reactpy/reactjs/module.py	Integrated file locking, added import_reactjs helper with framework selection
src/js/packages/@reactpy/client/src/bind.tsx	New file implementing framework detection and binding logic
src/js/packages/@reactpy/client/src/vdom.tsx	Refactored to use framework detection, fixed double-mount issues
src/js/packages/@reactpy/client/src/components.tsx	Fixed race conditions in binding lifecycle
tests/test_web/test_module.py	Made module names unique, added display fixture override, fixed browser parameter
tests/test_reactjs/test_modules_from_npm.py	Pinned package versions, converted to module-scoped fixtures, improved assertions
tests/test_reactjs/test_utils.py	Updated function names from resolve_from_* to resolve_names_from_*
pyproject.toml	Added parallel test support, reorganized build scripts, updated test configuration
.github/workflows/check.yml	Added --parallel flag to test commands