fix: #3321 validate prompt configs

[Aphroq]

Summary

• Validate prompt configs before forwarding them to model providers, so missing or non-string id values and invalid optional fields raise UserError.
• Apply the shared validation to static Agent prompts, dynamic prompt return values, and RealtimeAgent prompt settings while preserving existing valid prompt payloads.

Test plan

• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3321

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Validate the final realtime prompt setting

In RealtimeSession, this validates only the agent prompt before the already-supported run_config.model_settings / model_config.initial_model_settings prompt can remain in updated_settings or starting_settings can overwrite it later. For example, a session with initial_model_settings={"prompt": {}} and no agent prompt still forwards the invalid prompt to _get_session_config, where _passed_prompt["id"] raises KeyError instead of the new UserError; validate updated_settings["prompt"] after all realtime settings have been merged.

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