Introduce a GitHub Actions test matrix (fast and full gates) to run CI test groups. Add a Playwright-based browser smoke test (test/e2e/ui-smoke.test.js) that launches the app and verifies basic UI flows. Update README with local instructions for running e2e (install browsers) and the ci:fast/ci:full command groups.

Introduce launchBrowserOrSkip helper to centralize Chromium launch and skip logic when not installed, and refactor existing smoke test to use it. Add several end-to-end UI smoke tests: successful create submission (stubbing /create), client-side JSON validation that prevents network calls, query form results (stubbing /query), and overwrite conflict handling (stubbing /overwrite returning 409). Tests use Playwright routing to mock server responses and assert flash messages and object viewer output.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Capture fetch URL/options in test mocks and add assertions to verify the upstream RERUM contract (URL, HTTP method, Authorization header, Content-Type). Updates test/routes/{create,delete,overwrite,query,update}.test.js to record lastFetchUrl/lastFetchOptions and assert correct endpoint/method/headers, and documents the new validation in test/TESTING.md with an example and rationale to catch breaking changes.

Update test suites to accept application/ld+json by configuring express.json({ type: ['application/json', 'application/ld+json'] }) across create, query, delete, overwrite, and update tests. Add new tests: create and query now verify requests with Content-Type application/ld+json are accepted. Add tests that sending text/plain yields 415 in create, delete, overwrite, and update. Import and mount the error messenger in delete, overwrite, and update tests. Add network-failure cases that map rejected fetch to a 502 response for overwrite and update.

Add tests to exercise create route behavior: ensure request body id is converted to _id before upstream, and verify a successful upstream response missing id fields is treated as an error (502). Add new tests for verifyJsonContentType middleware to ensure it returns 415 when the Content-Type header is missing or when multiple Content-Type values are provided. These tests guard against regressions in id handling and content-type validation.

Add two tests to test/routes/error-messenger.test.js: one verifies the error messenger returns early when headers have already been sent (preserving partial response and leaving status 200), and another verifies that when an upstream error provides a text/plain Content-Type and a text() method, the messenger forwards the plain-text body and upstream status (example uses 418). These cover edge cases for header-sent behavior and handling of plain-text upstream responses.

Adds test/routes/rerum.test.js which verifies that fetchRerum maps fetch timeouts (AbortError) to a 504 upstream timeout error. The test mocks global.fetch to reject with an AbortError, sets RERUM_FETCH_TIMEOUT_MS to a low value, and asserts the returned error has status 504 and an appropriate message. Test setup/teardown restore the original global.fetch and environment variable.

Add a new test file for checkAccessToken (test/routes/tokens.test.js) to cover missing ACCESS_TOKEN, malformed JWTs, and refresh error propagation. Update error-messenger.test.js to assert statusCode/statusMessage fallback handling. Extend rerum.test.js to save/restore global setTimeout/clearTimeout and add tests that map non-timeout fetch failures to a 502 upstream error, ensure provided AbortSignal is forwarded, and verify fallback to the default timeout when the configured timeout is invalid.

Add unit tests for /query to cover upstream and network failure handling: (1) preserves upstream error text when fetch returns non-ok (503), (2) falls back to a generic RERUM error message when upstream .text() throws, and (3) maps rejected fetch (e.g., socket hang up) to a 502 response. Tests mock global.fetch and assert statusCode 502 and expected response text.

Add several unit tests to improve error handling coverage and token middleware behavior. create.test.js and overwrite.test.js gain tests that preserve upstream text on non-409 failures, fall back to a generic RERUM message when upstream .text() throws, and map missing-id successful payloads to 502. error-messenger.test.js replaces res.write with res.end in an existing test and introduces createMockRes plus focused messenger unit tests for headersSent, payload JSON, JSON content-type, and plain-text upstream errors. tokens.test.js adds tests ensuring valid or non-numeric-exp access tokens skip refresh. These changes increase robustness of error-path handling and token refresh logic in tests.