Fix null export (#69) and stale export after clear-retrace (#106)

[kellenmurphy]

This fixes two bugs that both cause the export to silently fail.

The first (related to #69) happens when a request is blocked, cancelled, or interrupted before a response arrives. getResponse() returns undefined, which gets passed to createFromJSON. Inside there, JSON.stringify(undefined) doesn't produce a string — it returns the JS value undefined — so JSON.parse throws a SyntaxError. The exception goes uncaught, ui.exportResult is never updated, and the downloaded file contains just null. The fix is a one-line null guard in enrichWithResponse in SAMLTraceIO.js.

The second (related to #106) happens when a request entry has isVisible set but parsed was never populated — this can occur when onHeadersReceived fires before onBeforeSendHeaders in a clear-and-retrace scenario. The export filter maps req.parsed across all entries, producing undefined in the array, which crashes createFromJSON and leaves ui.exportResult stale. The fix is .filter(Boolean) after the .map call in exportDialog.js.

Both fixes are covered by Jest unit tests and Playwright end-to-end tests. The Playwright suite lives on the playwright-testing branch at https://github.com/kellenmurphy/SAML-tracer/tree/playwright-testing — see E2E_TESTS.md there for context and instructions on running the tests. Each bug test was confirmed to fail on unfixed code and pass with the fix applied.

[tvdijen]

@khlr I think if you agree with the PR, we should merge and release a new version

[kellenmurphy]

Done, @tvdijen — bumped checkout, setup-node, and node-version per your suggestions. CI is green.

[khlr]

Thank you very much for this excellent contribution, Kellen! The fixes are so 'simple' – it's just crazy. But it's great that you found them.

I'm a huge fan of automated testing. Hence I'm delighted that, as well as carrying out the actual tests, you’ve also put the necessary infrastructure in place.
I’d actually thought about writing tests for SAML-tracer once before. In the end, though, my lack of experience with JavaScript testing and GitHub Actions put me off. It’s great that everything needed is now in place so we can easily add more tests in future!

[khlr]

I just published release 1.9.3 to AMO and CWS. After the store-reviews, the new version will be available in the respective store. Thanks again, Kellen!