ci: update test configuration to vendorize dandiapi instance in tests

[candleindark]

This PR is a duplicate of #355 since #355 was accidentally closed by mistake.

This PR updates test configuration in a GH workflow to vendorize the dandiapi instance used in tests. It builds on dandi/dandi-cli#1756, so it only works property after dandi/dandi-cli#1756 is merged.

This PR closes #350.

Notes:

• 1. For this PR to have any effect, Make dandi-archive instance used by tests vendorizable dandi-cli#1756 must be merged.
• 2. After Make dandi-archive instance used by tests vendorizable dandi-cli#1756 is merged, this PR is expected to fail until all more vendorization efforts for dandi-cli, , such as Review operation and codebase for further hardcoded DANDI identifiers dandi-cli#1752, are complete, and related changes are released.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.89%. Comparing base (58ccbca) to head (9ec7784).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #360   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files          18       18           
  Lines        2377     2377           
=======================================
  Hits         2327     2327           
  Misses         50       50           

Flag	Coverage Δ
unittests	97.89% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR updates the GitHub Actions test workflow to support vendorized dandiapi instances in tests. It adds configuration for testing against the EMBER-DANDI instance in addition to the default configuration, enabling the test suite to validate compatibility with different DANDI Archive instances.

• Adds a vendored_dandiapi matrix parameter with a default value
• Includes two new matrix combinations for testing with the EMBER-DANDI instance (for both master and release versions)
• Sets environment variables for instance name, identifier, and DOI prefix when custom instances are used

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[candleindark]

Notes:

As of now, only the integration tests against the release version of dandi-cli are failing. Once, the changes associated with dandi/dandi-cli#1752 are release, all tests should pass.

[candleindark]

@yarikoptic With the latest release of dandi-cli, all this one passes all tests, so this one is ready to merge.