fix: guard against nil onAck in result-returning *Async methods

[lmars]

Summary

• AppendMessageAsync(msg, nil) (and the UpdateMessageAsync / DeleteMessageAsync siblings) segfaulted when the server ACK arrived, because performMessageOperationAsync invoked the callback unconditionally at realtime_channel.go:908.
• The same latent bug existed in PublishWithResultAsync and PublishMultipleWithResultAsync.
• Normalize a nil onAck to a no-op at the top of each — mirroring the nil-safety that msgAckCallback.call already provides at the lower layer.

Surfaced by ably/docs#3351, where a fire-and-forget append example using AppendMessageAsync(msg, nil) crashed at runtime.

Test plan

• New integration sub-test TestRealtimeChannel_MessageUpdates/AppendMessageAsync/nil_onAck_is_treated_as_fire-and-forget — calls AppendMessageAsync(msg, nil), then round-trips a follow-up sync AppendMessage so we know the ACK for the nil-callback append landed without panicking the connection's read loop. Passes against nonprod:sandbox in ~0.6s.
• go vet ./ably/... clean
• go build ./... clean

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Async message publish operations now handle missing callbacks gracefully, enabling fire-and-forget usage patterns without errors or crashes. Improved stability for asynchronous publishing workflows.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: ecd18c44-c355-460e-88f9-729356e43272

📥 Commits

Reviewing files that changed from the base of the PR and between 439d9f3 and b9e6eeb.

📒 Files selected for processing (2)

• ably/message_updates_integration_test.go
• ably/realtime_channel.go

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Walkthrough

This pull request adds nil onAck callback tolerance to async message operations in RealtimeChannel. Three functions now install a default no-op handler when the callback is nil, enabling fire-and-forget usage without panic. A new integration test validates the safe behavior.

Changes

Nil acknowledgment callback handling

Layer / File(s)	Summary
Nil callback guard in async message operations ably/realtime_channel.go, ably/message_updates_integration_test.go	PublishWithResultAsync, PublishMultipleWithResultAsync, and performMessageOperationAsync now assign a default no-op onAck when the callback is nil, treating async publishes as fire-and-forget. A new subtest validates this behavior by calling AppendMessageAsync with nil callback and confirming the connection read loop processes the operation without panic.

🎯 2 (Simple) | ⏱️ ~8 minutes

🐰 A hop, skip, and fire away!
When callbacks are nil, no panic today
Async ops dance in a fire-and-forget way
The tests hop in to save the day
Safe and sound in every pathway! 🔥

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the primary change: fixing nil onAck handling in result-returning async methods to prevent segfaults.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/append-message-async-nil-callback

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