Expose standalone image generation in code mode

[won-openai]

Why

Standalone image generation remained top-level-only in code-mode sessions.

What changed

• Change imagegen exposure from DirectModelOnly to Direct.
• Keep direct-mode access while enabling nested code-mode access.
• Add a focused regression test for the exposure contract.

Validation

• just test -p codex-image-generation-extension

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add integration coverage for code-mode imagegen

This changes ImageGenerationTool from model-only to Direct, which changes agent tool planning by making image_gen.imagegen available through code-mode nested tools, but the commit does not add an integration test for that user-facing path. The repo's Test authoring guidance says, "Features that change the agent logic MUST add an integration test," so the code-mode exposure/dispatch contract can regress without coverage.

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