[Release gate]: Matrix live homeserver smoke check before v0.8.0 stable

[Audacity88]

Summary

Add a v0.7.6 release-gate smoke check for Matrix channel behavior after the matrix-sdk 0.17 bump landed in #6566.

Problem statement

#6566 merged the matrix-sdk 0.16 -> 0.17 compatibility bump plus the channel-matrix recursion-limit fix and low-memory install heuristic. The PR was reviewed and merged with green CI, but both the PR body and reviews explicitly noted that no live Matrix homeserver smoke test was run.

That is acceptable for merging the dependency/compile fix, but before shipping a release that includes the bump, we should verify the main runtime Matrix flows against a real homeserver so SDK API compatibility is not only compile/test-suite confidence.

Proposed solution

Before v0.7.6 ships, run a live Matrix smoke test on a real homeserver covering:

• send a normal message
• send or receive a threaded reply
• send a reaction
• send an attachment
• send or receive the voice path if the configured test setup supports it

Record the tested commit/version, homeserver type if relevant, and any known skips. If one of these paths fails, open or link a follow-up bug and decide whether it blocks the release.

Non-goals / out of scope

• No new Matrix feature work.
• No requirement that every Matrix dependency bump block on live homeserver smoke before merge.
• No requirement to commit credentials, homeserver URLs, tokens, or private room IDs.

Acceptance criteria

• A maintainer or reviewer records a v0.7.6 Matrix live smoke result after fix(channels,deps): bump matrix-sdk 0.16 → 0.17 #6566 is in the tree.
• The result covers send, threaded reply, reaction, attachment, and voice, or explicitly names skipped paths.
• Any failure is linked to a follow-up issue or called out as a release blocker/non-blocker.

Architecture impact

Matrix channel runtime validation only. No code architecture change is proposed by this issue.

Risk and rollback

Low implementation risk: this is a release validation task. The risk being managed is shipping matrix-sdk 0.17 without a real-homeserver sanity check.

Breaking change?

No

Data hygiene checks

• I removed personal/sensitive data from examples, payloads, and logs.
• I used neutral, project-focused wording and placeholders.