[aheritier]

Closes #4684. Follow-up from epic #4580.

What

This PR removes rulebased.Client.lastSelectedID and its LastSelectedModelID() accessor.

A provider client is built once per team load and shared by every conversation ("Client and Session Lifetimes" in AGENTS.md). So "the route chosen by the most recent request" mixed conversations. Since #4579, accounting and the UI read the per-stream ServingModel instead, and no production code read the accessor any more.

File	Change
pkg/model/provider/rulebased/client.go	Field, Store and accessor removed; the CreateChatCompletionStream doc now points to the stream's ServingModel
pkg/model/provider/rulebased/client_race_test.go	TestLastSelectedModelID_Concurrent becomes TestServingModel_ConcurrentRoutes: 100 concurrent requests on one router, each stream reporting its own route. Only the goroutine reading the removed accessor is gone
pkg/runtime/routing_pricing_test.go	One redundant assertion dropped from the fallback case: leaf.callCount == 1 beside it already proves the route was selected, and the persisted model and cost assertions cover pricing

Validation

• task build passes.
• golangci-lint v2.14.0 reports 0 issues, and go run ./lint . reports no offenses.
• go test -race passes on pkg/model/provider/rulebased, pkg/model/provider, pkg/runtime and pkg/tui/... (routing, pricing and sidebar).
• git grep "LastSelectedModelID\|lastSelectedID" -- '*.go' returns nothing.

[Copilot]

🟢 Approval recommended

The narrow cleanup leaves no dangling references and preserves per-stream identity handling and relevant regression checks.

No open issues 🎉

What changed in this PR

Removes obsolete client-wide routing state, leaving per-stream ServingModel as the source of routed model identity. This completes #4684 and follows the accounting changes in #4579.

Changes:

• Removes lastSelectedID, its accessor, and unused imports.
• Preserves concurrent per-stream route checks under a renamed test.
• Removes the obsolete accessor assertion while retaining fallback pricing checks.

File	Description
pkg/​runtime/​routing_pricing_test.go	Removes the obsolete assertion; retains route-count and pricing checks.
pkg/​model/​provider/​rulebased/​client.go	Removes shared last-selection state and documents per-stream identity.
pkg/​model/​provider/​rulebased/​client_race_test.go	Renames the concurrency test and removes accessor reads.

_{Review effort: Balanced}