feat: allow session config changes during a running turn

[testikun]

Summary

Fixes #3347.

Allow model, thinking-level, and permission-mode changes while a root Turn is running without mutating the active AgentRun.

Implementation

• Persist a Host-owned pendingConfiguration projection with the Session header.
• Keep the active AgentRun and its backend/execution boundary immutable.
• Merge successive client edits against the Host pending projection, including per-model thinking-level reset/restore behavior.
• Revalidate the pending connection, model, and thinking level at the next-Turn admission boundary; if validation fails, preserve the pending selection and report a typed error.
• Apply the pending configuration at the fresh root-Turn admission boundary, then clear it before creating the successor AgentRun.
• Project pending state through the Runtime Host Session catalog so reconnects, restarts, and multiple clients converge.
• Keep Plan, Swarm, Graph, workspace, and Side Chat configuration outside this slice.

Tests

• Runtime staging, next-admission application, and failed-validation preservation regression coverage.
• Runtime Host model-readiness and thinking-level revalidation coverage.
• Desktop Host-client pending merge and model/thinking reset coverage.
• Full npm test: 1403 passed.
• Full lint, format, typecheck, and git diff --check verification passed.