[4 of 7] Remove core OverrideTurnContext op

[etraut-openai]

Stack position: [4 of 7]

Summary

This PR removes the legacy Op::OverrideTurnContext which has no production callers. The next PR will introduce a new Op::TurnCodex which has similar, but not identical, semantics to the legacy op.

Changes

• Delete Op::OverrideTurnContext from the core protocol and submission loop.
• Move tests that used that op onto the existing thread-level turn-context override shape.
• Keep the new queued context-only op out of this PR; that is introduced in the next stack layer.

This stack addresses #20656 and #22090.

Stack

1. [1 of 7] Add turn context to UserInput
2. [2 of 7] Remove UserInputWithTurnContext
3. [3 of 7] Remove UserTurn
4. [4 of 7] Remove core OverrideTurnContext op (this PR)
5. [5 of 7] Add queued core turn context op
6. [6 of 7] Add app-server next-turn state API
7. [7 of 7] Sync TUI next-turn state

[chatgpt-codex-connector]

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