Initialize EAM /appinstances callback (2)

[clundie-CL]

What type of PR is this?

enhancement/feature

What this PR does / why we need it:

This PR continues work from #373 (originally opened by @travishepworth ) to add callback support for POST /appinstances. Travis's PR was opened from a fork on his personal GitHub account and since I don't have permissions to push commits to this repo, I have elected to recreate his branch on the CableLabs' fork and submit this companion PR.

This update addresses reviewer feedback (#373 (comment)) by expanding the terminationReason enum to include comprehensive termination scenarios that prevent infinite retry loops and provide developers with actionable diagnostic information.

Key additions:

• Adds CAMARA-standard termination reasons: SUBSCRIPTION_DELETED, NETWORK_TERMINATED
• Adds application-specific failure reasons: DELIVERY_FAILURE (for repeated sink endpoint failures), RESOURCE_DELETED (for deleted app instances/deployments)
• Adds inline documentation describing each termination reason

This aligns with CAMARA CloudEvents standards and webhook API best practices, ensuring developers receive clear signals about why subscriptions terminated and whether they should retry.

Which issue(s) this PR fixes:

Fixes #370

Special notes for reviewers:

This PR addresses the review feedback from #373 regarding the need for expanded termination reasons with diagnostic information. The termination reasons now follow the CAMARA CloudEvents subscription template standard and include reasons that prevent clients from retrying operations that will always fail due to application-level issues.

The implementation remains backward-compatible. No breaking changes to existing callback behavior.

Changelog input

Add callback response support to the EAM POST /appinstances endpoint with comprehensive subscription termination reasons following CAMARA standards

#### Additional documentation

Updated terminationReason enum in CloudEvent schema with seven comprehensive termination scenarios including SUBSCRIPTION_EXPIRED, MAX_EVENTS_REACHED, ACCESS_TOKEN_EXPIRED, SUBSCRIPTION_DELETED, NETWORK_TERMINATED, DELIVERY_FAILURE, and RESOURCE_DELETED. Each reason includes inline documentation for developer clarity.

[JoseMConde]

Hi @hdamker , do you know why the megaLinter stopped after 15 minutes??
Thanks!!

[hdamker]

Hi @hdamker , do you know why the megaLinter stopped after 15 minutes??
Thanks!!

The question is rather why it ran 15 minutes, that's much too long ... that can be a timeout.
Yesterday evening had GitHub a major outage of their runners ... have you checked the GitHub status?

Just restarted the job and it failed after a minute ;-)

[clundie-CL]

@hdamker Thanks for taking a look and providing some insights.

@JoseMConde and @gunjald PR should be ready now that the linter error has been addressed.

[gunjald]

I think as an enhancement of the documentation we need to slightly update the description which is not linked with this issue itself. The terms like application deployment, application onboarding and application instantiation seems to have some overlaps which can be improved. So we may have another issue created to take the discussion outside of this issue.

[gunjald]

Should we allow for some max limit that the platform can impose? Otherwise I may put an arbitrary big number which could mean unlimited events?