fix(v2): preserve running turns during provider handoff

[Bil0000]

What remains after rebasing

#12226 is now in this PR's base branch. Its model-selection events, premature client-update removal, and queued cross-provider handoff stay in that base; the overlapping commits and replay were dropped from this PR.

This PR keeps distinct handoff fixes:

• Imported threads can offer a provider handoff before a provider session exists, while a preparing turn remains bound until its session appears; session capabilities control established turns.
• Automatic messages queue while the active handoff run is preparing or starting.
• Handoff rows show provider logos and model names, with account names available on hover or tap.
• Provider failure text uses an explicit safe message or the normalized default instead of arbitrary nested cause text.

Evidence

After the rebase, 129 focused tests across the changed server, mobile, and web paths passed. Server, web, mobile, and client-runtime typechecks passed. Formatting and lint on the 15 changed files passed. All running CI checks passed on the rebased head; the approvability check was skipped.

Before:

After:

Narrow viewport:

Model: GPT-6. Harness: Codex.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR changes provider-handoff eligibility and message dispatch ordering across shared runtime, server, web, and mobile code. It also changes persisted provider failure text to suppress arbitrary cause data, making the cross-cutting runtime and sensitive error-data impact appropriate for human review.

^{You can add or adjust custom eligibility rules. Learn more.}

Dismissing prior approval to re-evaluate 3b01afe

Dismissing prior approval to re-evaluate 11dae2c

Dismissing prior approval to re-evaluate 183861d

Dismissing prior approval to re-evaluate 9b2a1cd