fix(agentic): preserve background subagent dialog turn ids

[wsp1911]

Summary

Fixes a background subagent regression introduced by 5e48999f94daf119c1217ed6b71ba878d564f5dd (refactor(cli): route Peer Host through the assembled runtime).

• Allocate the child dialog_turn_id before composing background-result metadata.
• Reuse that ID in the scheduler and direct hidden-subagent paths.
• Generate new child turn IDs as raw UUIDs rather than subagent-<uuid>.
• Add coverage for raw UUID generation and preallocated-ID reuse.

Fixes #

Type and Areas

Type: Regression fix

Areas: Rust core, agent coordination, subagent scheduling

Motivation / Impact

5e48999 added exact parent/child turn lineage to background-result metadata for Peer Host routing and delivery control. It also read request.dialog_turn_id before the scheduler or direct execution path assigned it.

The prepared hidden-subagent request intentionally has no turn ID at that stage, so every background subagent failed with:

prepared hidden subagent request is missing dialog_turn_id

The fix establishes the child turn ID at the background-launch boundary and preserves it through scheduling and execution. This keeps metadata, the scheduler queue, and the actual child DialogTurn aligned.

No migration is required. Existing persisted turns remain unchanged.

Verification

• pnpm run fmt:rs
• git diff --check
• cargo check -p bitfun-core -j 1
• cargo test -p bitfun-core hidden_subagent_dialog_turn_id_reuses_existing_or_generates_raw_uuid -- --nocapture
• cargo test -p bitfun-core --lib hidden_subagent_dialog_turn_id_reuses_existing_or_generates_raw_uuid -j 1 -- --nocapture
• Manual: reproduced the original background-subagent failure, then confirmed the same scenario succeeds after the fix.

Reviewer Notes

The regression came from moving the requirement for a child turn ID ahead of its existing allocation point. The scheduler now reuses a preallocated ID, and the non-scheduler path already consumes a supplied ID, so both execution modes retain one stable identity for Peer Host lineage and background-result delivery.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, with local test limitations explained.
• No user-facing strings, docs, or locale resources changed.