Auto-save credentials to cache on successful auth

[KCarretto]

Updates the tavern CLI auth package to automatically write a retrieved API key to the configured cache file if authentication was successful, creating the directory if necessary. Tests have been expanded to verify this behavior.

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PR created automatically by Jules for task 4494069843278243605 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2743    ±0	2743    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1532	✅	2743	2743	0	0	0	0	38.6s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2743	0	0	5.7s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
TestDockerExecutor_Build_ContextCancellation	1	5.7s	5.7s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
TestInteractiveShell	1	5.0s	5.0s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.8s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.8s
imix::bin/imix: install::tests::test_install_execution	3	1.6s	4.8s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚