smart_routing: make provider pinning configurable

[bussyjd]

Problem

smart_routing currently pins provider/model in context once selected. This is useful for tool loops but can be too sticky for reused contexts.

Current behavior

• Pinning reuses prior selection when _smart_routing_selection exists in context.
• A low-complexity first message can lock model/provider choice for later higher-complexity prompts if context is reused.

Proposal

Add configurable pinning mode:

• off: never pin
• tool_loop: pin only within a single request/tool-execution loop (recommended default)
• request: pin for whole request context (current behavior)

Config shape:

{
  "overrides": {
    "pinningMode": "tool_loop"
  }
}

Acceptance criteria

• Default behavior is tool_loop.
• Existing behavior preserved when pinningMode=request.
• pinningMode=off always re-routes per call.
• Routing metadata reports active pinning mode and whether a response used pinned selection.

Tests

• No pinning: provider can change between sequential calls.
• Tool-loop pinning: provider remains stable across tool-call iterations.
• Request pinning: current sticky behavior preserved.
• Invalid config value falls back to default safely.