Preserve connecting status while a turn starts

[D3OXY]

What Changed

Preserve the connecting thread status while provider readiness arrives with a turn start still pending. The thread transitions directly to working once the turn starts instead of briefly appearing inactive.

Why

Status-grouped thread lists currently jump between sections during provider startup because readiness temporarily exposes an idle state.

Fixes #4099

Verification

• vp check
• vp run typecheck
• vp test: lifecycle coverage passed; the unrelated ACP timeout passed 14/14 on targeted retry.

Checklist

• This PR is small and focused
• I explained what changed and why
• UI screenshots included
• Video included

Note

Preserve 'starting' session status while a turn start is pending

• When thread.turn.start is handled, the session is immediately projected as starting before the provider session is fully ready, and remains starting even if the provider reports ready until a turn actually begins.
• ProviderRuntimeIngestion maps session.state.changed 'ready' and session.started events to starting (rather than ready) when a pending turn-start exists for the thread.
• On thread.session-set with terminal statuses (error, stopped, interrupted), any pending turn-start records for the thread are deleted via projectionTurnRepository.deletePendingTurnStartByThreadId.
• setThreadSessionErrorOnTurnStartFailure now constructs a minimal session from the thread when none exists, setting status to error (or keeping stopped) so turn failures are always surfaced.
• Behavioral Change: sessions that previously became ready between turn-start and the first turn event will now show starting instead.

^{Macroscope summarized 919bd72.}

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Approvability

Verdict: Needs human review

This PR modifies session state machine behavior in the orchestration layer, changing how status transitions during turn startup. The changes affect core runtime state management across multiple files, warranting human review despite good test coverage.

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Dismissing prior approval to re-evaluate 1c1aa71