Avoid duplicate planner failure cards

[esokullu]

Summary

• keep planner_request_failed warnings as the single actionable Retry/Providers presentation
• prevent reconnect snapshot recovery from synthesizing a second generic error for the same planner failure
• preserve generic error synthesis for ordinary failed snapshots and existing error events
• add mirrored Chrome and Firefox regression coverage

Root cause

Planner request failures are recorded as actionable warning events, while their terminal run snapshot is marked failed. runResponseFromSnapshot synthesized a generic error for every failed snapshot without an explicit error event, so the side panel rendered both the planner warning card and a second generic error card.

User impact

A failed planner request now shows one actionable error card instead of two duplicate presentations. Other failure types retain their existing fallback error behavior.

Validation

• node --check src/chrome/src/run-reconnect.js
• node --check src/firefox/src/run-reconnect.js
• new Chrome/Firefox planner snapshot regression passes
• npm run test:security — 60/60 checks passed
• main suite — 1,432 passed; one unrelated existing failure remains because package.json is 26.0.10 while the changelog latest entry is 26.0.0