fix(tui): keep session model stable when recents sync - #52533

Found a case this doesn't cover (reported in #54375). The fallback is captured per agent the first time that agent is resolved in a session, so an agent that hasn't been selected in the session yet (for example after the TUI restarts) still picks up a model chosen meanwhile in another tab or client:

1. Session A: the last prompt was sent with plan on model X; build has no model and there is no global model in config.
2. In another tab, pick model Y.
3. Back in A, switch to build: it runs on Y, though nothing was picked in A.

test("an agent first shown after another tab picks a model keeps the session fallback", async () => {
  await using setup = await renderLocal({
    models: [model("first"), model("second")],
    agents: [agent("build"), agent("plan")],
    preferences: { recent: [{ providerID: "provider", modelID: "first" }] },
    sessions: [
      session("ses_a", { providerID: "provider", id: "first" }, "plan"),
      session("ses_b", { providerID: "provider", id: "first" }, "build"),
    ],
  })
  await Promise.all([setup.data.session.sync("ses_a"), setup.data.session.sync("ses_b")])
  setup.route.navigate({ type: "session", sessionID: "ses_a" })
  setup.route.navigate({ type: "session", sessionID: "ses_b" })
  setup.local.model.set({ providerID: "provider", modelID: "second" }, { recent: true })
  setup.route.navigate({ type: "session", sessionID: "ses_a" })
  setup.local.agent.set("build")
  expect(setup.local.model.current()?.modelID).toBe("first")
})

Capturing the fallback for all agents when the session is first resolved would make the test pass, but it would still snapshot the shared recents at that moment rather than use the session's own model. Which model build should get here is the open question in #54375 and #43179, so leaving it out of scope is fine; just flagging the gap.

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