feat(core): add session model.select hook for per-step model routing - #50965

The following comment was made by an LLM, it may be inaccurate:

This hook appears to provide the missing pre-resolution seam for the Jev router in this task. One additional contract matters for prompt-based routing: pick the model for a newly admitted user turn, retain that choice for the turn's tool continuations/retries/compaction recovery, and resolve a queued user turn independently when it begins. The current step field may provide that boundary, but a test should establish its behavior when prompts queue during a tool wait and when a request retries.

There is also a feedback requirement: plugins need the effective per-turn model reflected in the TUI and desktop while preserving the user's persistent session default. Since this PR intentionally leaves the Session's selected model unchanged, could the PR document how clients observe the effective model for each response, or add that signal if it is not already available?

A regression test with prompt A -> tool call -> queued prompt B would help: A's continuation must use A's selected model, then B must independently select its own model. Retrying/compaction should retain A's model. This request is specifically to preserve one choice per user prompt; automatic re-selection on each continuation would change the intended Jev routing behavior.

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