app-server: persist conversation modalities + backfill on entry

[ccy-oai]

Summary

• Persist conversation modalities per thread in the state DB.
• Backfill modalities on first entry (thread/start, thread/resume, thread/fork) when NULL.
• Surface conversationModalities on v2 Thread responses.

Rationale

• Clients need O(1) signal of whether a conversation ever used images.
• Backfill is one-time and
  avoids repeated history scans.

Scope

• App-server + state DB schema.

Testing

• Not run (manual build OK on local)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid hard-failing on deferred rollout during thread start

This new read_summary_from_rollout call makes thread/start fail when the rollout file has not been materialized yet, but new sessions intentionally defer file creation until an explicit persist (the core rollout recorder documents this behavior, and record_initial_history for InitialHistory::New only flushes). In that case this branch returns an internal error even though the thread was already spawned, so clients see a failed start while a live thread remains in memory; the same pattern is also used in resume paths. Please fall back to snapshot-derived thread metadata when the rollout is not yet present instead of treating it as a fatal error.

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