fix(callbacks/v2): OnTimeoutPacket blocked by UnmarshalPacketData error (backport #8856)

[mergify]

OnTimeoutPacket propagates UnmarshalPacketData errors, blocking the packet timeout lifecycle. This is inconsistent with OnSendPacket (line 124) and OnAcknowledgementPacket (line 254), both of which return nil on the same error, allowing the packet lifecycle to continue unblocked.

The inconsistency is also visible in the comments:

• OnSendPacket: "is not blocked if the packet does not opt-in to callbacks" (line 123)
• OnAcknowledgementPacket: same comment (line 252)
• OnTimeoutPacket: comment was missing (now added)

The V1 callbacks middleware does not have this inconsistency because it uses a different pattern (GetSourceCallbackData combining unmarshal and callback data extraction).

Impact: If a packet's payload cannot be unmarshalled by the callbacks middleware (e.g., codec version mismatch), the timeout transaction reverts entirely. The underlying app's OnTimeoutPacket has already executed the refund (line 317-320), but the revert rolls back all state changes. Since the packet has already timed out on the destination chain and cannot be received, the sender's funds become permanently locked with no recovery path.

This aligns with the principle stated at line 359: "callback execution errors are not allowed to block the packet lifecycle."

---

This is an automatic backport of pull request #8856 done by Mergify.

[mergify]

Cherry-pick of 0e8a117 has failed:

On branch mergify/bp/release/v10.5.x/pr-8856
Your branch is up to date with 'origin/release/v10.5.x'.

You are currently cherry-picking commit 0e8a1170.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   modules/apps/callbacks/v2/ibc_middleware.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[greptile-apps]

PR author is not in the allowed authors list.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/v10.5.x@c990bce). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
modules/apps/callbacks/v2/ibc_middleware.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##             release/v10.5.x    #8911   +/-   ##
==================================================
  Coverage                   ?   62.54%           
==================================================
  Files                      ?      288           
  Lines                      ?    20812           
  Branches                   ?        0           
==================================================
  Hits                       ?    13016           
  Misses                     ?     7230           
  Partials                   ?      566           

Flag	Coverage Δ
08-wasm	66.28% <ø> (?)
e2e	1.18% <ø> (?)
ibc-go	68.96% <50.00%> (?)

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