fix: stabilize session/create flow — runtime-prefs handshake, third-party providers, lazy sessions, model state

[william0wang]

Summary

Fixes the session/create flow for editor-initiated sessions and the model
state that follows it: the backend handshake no longer blocks creation,
third-party providers load correctly, backend sessions are created lazily on
first use, and the editor's model/mode/thought UI tracks the session
authoritatively — including mid-turn switches — without drifting back to
defaults.

Changes

• Auto-reply session/requestRuntimePreferences (0a57cd5) — the backend
  sends this request during session/create; the bridge now answers it
  instead of letting session creation block.
• Load third-party providers via workspace/updateProviderRegistry
  (5ab6b6c) — the V4 backend no longer auto-loads providers from
  config.json; the bridge mirrors the host's ModelProviderService by pushing
  the registry (inline apiKey, apiFormat derived from kind), so
  third-party models no longer fail with provider_not_configured.
• Defer session/create until first use (90a890d) — session/new
  returns a placeholder; the backend session materializes lazily on the first
  prompt/resume/load, so an editor startup that never prompts leaves no empty
  sessions behind.
• Propagate mid-turn model/mode/thought switches via state.updated
  (7a4185a) — pushes config_option_update (+ current_mode_update)
  immediately instead of waiting for turn completion.
• Resolve lazy placeholders on session/resume / session/load
  (b4292d8) — keeps the acp_sid ↔ zcode_sid mapping consistent.
• Keep the selected model in the dropdown (8a19626) — a state.updated
  patch only carries changed fields; config options are now rebuilt from the
  session's authoritative settings instead of defaults, so a mid-turn
  mode/thought update no longer resets the model/mode/thought dropdowns to
  their default values.

Testing

• pnpm test — 360 tests pass (23 files); typecheck and lint clean.
• New coverage: provider-registry, lazy-sessions, session-lazy,
  runtime-model, dispatch (ConfigChanged model preservation), bugfixes,
  backend.

Docs

• docs/ARCHITECTURE.md and docs/PROTOCOL.md updated for the lazy-session
  lifecycle and the provider-registry push.