Add dry-run validation step to publish pipeline to prevent partial failures

[petehauge]

Problem

When apiops publish fails mid-execution, it can leave the Azure API Management instance in a partially updated (bad) state. This has been reported by a customer.

Proposed Solution

Add a dry-run step to the publish pipeline scaffolded by apiops init:

1. Before the actual publish, run a dry-run validation pass that checks whether all operations would succeed.
2. If the dry-run fails, halt the pipeline immediately — do not proceed to the actual publish step.
3. This ensures APIM is never left in an inconsistent state due to a failed publish.

Acceptance Criteria

• The publish pipeline generated by apiops init includes a dry-run stage/step before the actual publish
• If the dry-run step fails, the pipeline stops and does not execute the real publish
• Clear error output is provided when the dry-run fails, indicating what would have gone wrong
• Documentation is updated to explain the dry-run behavior