feat(delivery): add maximum backoff duration

[kahirokunn]

This PR implements the Alpha stage of #9278.

TL;DR: New experimental feature allowing users to opt in to capping normal delivery retry intervals calculated from backoffDelay and backoffPolicy.

Proposed Changes

• 🎁 Add the Alpha delivery-backoff-max feature flag, disabled by default.
• 🎁 Extend DeliverySpec with the optional backoffMax field.
• 🎁 Validate that backoffMax is a positive ISO 8601 duration and reject the field while the feature is disabled.
• 🎁 Propagate backoffMax through Subscription reconciliation and Channel delivery defaults.
• 🎁 Parse backoffMax in RetryConfigFromDeliverySpec() and apply it to linear and exponential backoff calculations.
• 🐛 Prevent linear and exponential backoff calculations from overflowing time.Duration, including when no explicit maximum is configured.
• 🎁 Keep delays requested through Retry-After independently controlled by retryAfterMax.
• 🎁 Add unit and experimental E2E coverage for capped delivery retries.
• 🎁 Update generated API documentation; user-facing documentation is tracked in DeliverySpec.BackoffMax Experimental Feature docs#6686.

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature (Planned if the experimental feature graduates to Stable.)
• Conformance test for any change to the spec (To be evaluated when the feature graduates to Stable.)

Release Note

The new experimental `delivery-backoff-max` feature flag lets you set `DeliverySpec.backoffMax` to cap normal retry intervals calculated from `backoffDelay` and `backoffPolicy`. The feature is disabled by default.

Docs

📖 knative/docs#6686

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

❌ Patch coverage is 89.09091% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.15%. Comparing base (72ec4f4) to head (93bc77a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kncloudevents/retries.go	86.04%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9279      +/-   ##
==========================================
+ Coverage   51.06%   51.15%   +0.08%     
==========================================
  Files         411      411              
  Lines       22120    22172      +52     
==========================================
+ Hits        11295    11341      +46     
- Misses       9951     9954       +3     
- Partials      874      877       +3     

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[kahirokunn]

/ok-to-test

[dsimansk]

/ok-to-test

shouldn't be needed anymore for you as an org member :)

[kahirokunn]

@dsimansk Thank you!
If you don't mind, could you please review this PR? 🙇‍♂️
I think backoffMax is a very basic tool that offers the essential features any valuable tool should have. 💪

[dsimansk]

@dsimansk Thank you! If you don't mind, could you please review this PR? 🙇‍♂️ I think backoffMax is a very basic tool that offers the essential features any valuable tool should have. 💪

Agreed, I've added one suggestion. Otherwise looks good, given it's feature flag guarded and well test covered.

[kahirokunn]

@dsimansk Thanks for the suggestion, I've addressed it! Could you take another look when you get a chance? 🙏

[kahirokunn]

/test upgrade-tests

[dsimansk]

@Cali0707 or @creydr any change you could chime in please?