Add E2E coverage for optimistic reaction set rollback

[dnlbui]

Summary

Add automated coverage in Liberdus/client-testing for the optimistic reaction set rollback flow implemented on web-client-v2 branch phase-8-reaction-set-rollback.

Reference PR:

• Phase 8/9: optimistic reaction set fallback and pending cleanup web-client-v2#1237

Scope

We want E2E coverage for the phase 8/9 reaction behavior on that branch:

• optimistic outgoing reaction set shows immediately
• immediate inject failure rolls the reaction back to the previous emoji
• pending explicit failure rolls the reaction back to the previous emoji
• pending timeout rolls the reaction back to the previous emoji
• pending success keeps the new emoji and removes the hidden fallback state
• reaction remove remains non-optimistic

Suggested Test Setup

Use a message that already has an existing reaction from the sender, for example:

1. sender reacts 😀
2. sender changes reaction to 🔥

That makes the expected behavior easy to verify:

• optimistic state: 🔥
• failure rollback: 😀
• success state: 🔥

What To Verify

From the UI perspective, verify:

• message chip shows the effective emoji only
• reaction picker active state matches the effective emoji
• chat-list preview/order updates correctly for reaction activity
• failure paths visibly revert to the old reaction
• success path leaves the new reaction in place

If the client-testing harness supports test hooks or network stubbing, cover these result paths:

• inject failure
• pending receipt success false
• pending receipt timeout / no transaction
• normal success

Notes

The web-client-v2 branch also has a local testing note and optional testing patch here:

• data_structures_flow/phase-8-9-reaction-testing.md
• data_structures_flow/phase-8-9-reaction-testing.patch

Those files are not intended as production changes, but they describe the exact scenarios and expected outcomes.