ci-automation: Report missing Azure product and plan lookups

[Shivanshu-GITH]

Fix

get_product_durable_id and get_plan_durable_id in
ci-automation/release/azure_marketplace_publish.py index the product-ingestion response
with [0] straight after .get("value", []):

return resp.json().get("value", [])[0].get("id")

The .get("value", []) default anticipates a missing key, but the index runs either way, so
a response with no results fails with a bare IndexError: list index out of range. During a
release publish that means an operator gets a traceback instead of a message saying which
offer or plan was not found.

This checks the list before indexing and raises with the externalId that was looked up. It
also calls raise_for_status(), so an HTTP error is reported as a request failure rather
than reaching the same line as an empty body.

get_image_versions() a few lines below already uses the safe .get form, so this brings
the two lookups in line with the rest of the file.

Reported in flatcar/Flatcar#2309.

How to use

No change in behaviour on the success path — the functions return the same durable id as
before. The difference is only visible when a lookup comes back empty or the request fails,
where the script now stops with a named error instead of an IndexError.

Reviewers may want to confirm the choice of raising over returning None: the caller does
product_durable_id.split("/")[1] immediately afterwards, so returning None would move the
failure one line down rather than remove it. Happy to switch to a logging.error and
continue in the caller loop instead, matching the generate_az_sas_url case, if that fits
better.

Testing done

I do not have Azure marketplace credentials, so the script has not been run against the live
Microsoft product-ingestion API. Instead I loaded the two patched functions directly and
stubbed only requests and the azure SDK, so the logic under test is the shipped logic:

PASS  empty product list: ValueError: no product found for offer externalId=flatcar-container-linux-free
PASS  missing value key:  ValueError: no product found for offer externalId=does-not-exist
PASS  empty plan list:    ValueError: no plan found for externalId=stable under product product/123
PASS  http 500:           HTTPError: 500 Server Error
PASS  populated product:  'product/abc123'
PASS  populated plan:     'plan/def456'

6/6 checks passed

The first three cases are what raised IndexError before:

$ python -c "{'value': []}.get('value', [])[0].get('id')"
IndexError: list index out of range

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)

  Not added — changelog/README.md says entries should only contain information relevant to
  end users, and this is internal release automation. Happy to add one if you would rather
  have it recorded.

• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

  Not applicable — no image content is touched.

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An LLM was used in producing this change.