fix: improve import service E2E test diagnostics

[blefebvre]

• The issue this PR is addressing is that failed E2E tests are difficult to debug, as the logs reveal little about the failure. Before:

AssertionError: expected false to be true

• With this change:

AssertionError: Expected 202 Accepted but got 429: Too Many Requests: API key hash b3523cc4472ccdfa__example__58de2066caed04e cannot be used to start any more import jobs for [https://implementationdetails.dev:](https://implementationdetails.dev/) expected false to be true

• Include HTTP status and x-error header in assertion failure message so the root cause of import job creation failures is visible in CI output
• Add USER_API_KEY env var to e2e workflow so suggestions-projection tests are no longer silently skipped when the secret is configured

Please ensure your pull request adheres to the following guidelines:

• make sure to link the related issues in this description. Or if there's no issue created, make sure you
  describe here the problem you're solving.
• when merging / squashing, make sure the fixed issue references are visible in the commits, for easy compilation of release notes

If the PR is changing the API specification:

• make sure you add a "Not implemented yet" note the endpoint description, if the implementation is not ready
  yet. Ideally, return a 501 status code with a message explaining the feature is not implemented yet.
• make sure you add at least one example of the request and response.

If the PR is changing the API implementation or an entity exposed through the API:

• make sure you update the API specification and the examples to reflect the changes.

If the PR is introducing a new audit type:

• make sure you update the API specification with the type, schema of the audit result and an example

[github-actions]

This PR will trigger a patch release when merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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