fix(chat): admit messages while sessions start

[loganj]

🤖

Summary

• Lets the composer accept a message into a draft chat's queue while its backend session is still being prepared.
• Keeps dispatch blocked until session preparation completes, including Agent Builder draft identity migration.
• Preserves failed-creation, read-only, Agent Builder preparation, and workspace preparation blocks.

This is PR 1 of 2. PR #65 updates the chat laws on top of this behavior.

Adapted from chailandau's squareup/berd#1076 for block/berd, while retaining the dispatch ownership introduced by block/berd#61.

Related issue

None found. Related pull requests: squareup/berd#1076 and #61.

Testing

• just check
• just test — 6,308 passed, 1 skipped
• Pre-push fmt, frontend check, Tauri check, and Clippy gate
• Focused queue-admission, promotion/migration interleaving, failure, and draft-preservation regressions

[morgmart]

🤖 Automated code review

REQUEST_CHANGES. Static review of the exact three-dot comparison found two blocking Agent Builder migration defects. Final self-check covered pending-session composer admission, ordinary and Agent Builder queue dispatch, draft-ID promotion, creation and migration failure, newer-draft preservation, navigation/consent guards, accessibility, i18n, async never-completes and race behavior, lifecycle, test honesty, design-system/project laws, duplicate overlap, and blocking user-effect evidence. The supplied GitHub evidence was inspected: all 8 captured check runs completed successfully; no local execution was performed. The findings are distinct: one concerns concurrent write/data-loss behavior, while the other concerns adopting a path returned by a successful rename. No review threads were supplied, so no findings were suppressed as duplicates.

Deterministic publication result: 2 blocking and 0 non-blocking finding(s) publishable; 0 duplicate(s) suppressed.

[loganj]

Changes requested here #64 (review) are addressed down-stack in PRs #68 and #78

Both P1 findings were confirmed as real at this PR's head, then resolved structurally by the stacked follow-up work: the defer-prep stack deletes migratePendingDraftAgent entirely (zero references remain), and the full stack is flattened into a single change before anything merges, so no unpatched interim window exists. Details in the resolved review threads. Dismissing on Logan's instruction so the stack can proceed.