fix: preserve Slides model routing across nested agents

[nicko-ai]

Summary

Fix Slides generation when nested planner and HTML-writer agents use an inherited or non-default model.

The previous implementation duplicated model setup across Insert and Modify flows. It could ignore RunConfig.model, select an unrelated fallback, and classify generic OpenAI-compatible endpoints as Codex Responses. That could produce failed or empty Slides output.

Changes

• Add one shared model-resolution and transport helper for both Slides tools.
• Resolve the request model first, then the caller model or wrapper, then the default.
• Use Responses only for verified Codex endpoints.
• Preserve direct OpenAI, compatible Chat Completions, OpenRouter, LiteLLM, Ollama, wrapper credentials, and request-level model overrides.
• Recover streamed text when the structured final response is empty.
• Add focused regression coverage for both Insert and Modify flows.

Validation

• Focused Slides tests: passed
• Ruff lint and focused formatting: passed
• Python compilation check: passed
• Git diff checks: passed
• Independent review: clean

Remaining acceptance

Hosted checks will run on this PR. A credentialed live gpt-5.4-mini Insert and Modify flow is still required before merge.

This replaces the current-main-relevant parts of #68 without carrying forward its stale and conflicting changes.