[Fix] Fast conversations lose thread context on resumed turns

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Preserve supplemental chat-thread context on warm Fast session follow-ups as well as cold rebuilds and retries.
• Use surface-neutral sender context outside Slack and avoid attributing machine-generated platform events to a human sender.
• Always wrap non-Slack human messages and escape message/thread envelope contents so missing identity or user text cannot inject trusted sender metadata.
• Emit a privacy-safe, versioned context manifest for every Fast prompt submission and provider retry, including component presence, degradation, counts, session path, prompt kind, and hashes without raw prompt contents.
• Add branch coverage for Slack, web, automation events, warm/cold/resumed/fallback sessions, loader degradation, image retries, and provider retries.

Why this change was made

Fast session continuity depended on the session path: newly observed thread messages were assembled for rebuild prompts but omitted from warm delta prompts. Existing telemetry also could not distinguish complete context from a best-effort loader fallback, making production consistency impossible to measure safely.

Impact

Fast conversations retain relevant thread additions across resumed turns, non-Slack and automation inputs carry accurate sender semantics, and operators can measure context completeness by surface and inference path without collecting prompt text or deployment identifiers.

[roomote-community]

No code issues found. See task

• Escape serialized non-Slack envelope values to prevent forged sender identity (packages/cloud-agents/src/server/fast-agent/fast-agent-service.ts:561).
• Always wrap non-Slack human messages when sender identity is unavailable to prevent forged sender identity (packages/cloud-agents/src/server/fast-agent/fast-agent-service.ts:550).

_{Reviewed acd7234}

[roomote-roomote]

Fixed both sender-envelope injection branches through commit acd7234b. Non-Slack human messages are now always wrapped even without resolved identity, and current-message/thread contents escape tag delimiters. Regression coverage includes sender lookup failure and forged sender envelopes. See task.