Fix: add SD negative prompt graph wiring and history

[DustyShoe]

Summary

Fixes SD negative prompt graph wiring and prompt history behavior.

SD1/SD2, SDXL, SD3, and multidiffusion upscale graphs now create a dedicated negative prompt string node and wire it into negative conditioning and negative_prompt metadata. The enqueue batch config also batches the negative prompt node using the preset-modified negative prompt, matching the positive prompt node flow.

Prompt history now stores positive and negative prompts together in one history item. Reusing a history item restores both prompts, search matches both fields, and old string-only prompt history entries are migrated to the new pair shape.

Related Issues / Discussions

N/A

QA Instructions

QA Instructions

1. Select an SD1.5 or SDXL model.
2. Enter a positive prompt and a negative prompt.
3. Generate an image.
4. Open the generated image in the workflow editor and verify the negative prompt string node is wired to the negative conditioning input and the metadata negative_prompt input connector.
5. Repeat with multidiffusion upscale and verify the negative prompt string node is wired to the negative conditioning input and the metadata negative_prompt input connector.
6. Open prompt history from the positive prompt field.
7. Verify one history entry shows both Prompt: and Negative Prompt:.
8. Click the history entry and verify both positive and negative prompt fields are restored.
9. Use Alt+Up/Down in the positive prompt field and verify both prompts are restored together.

Merge Plan

No special merge plan. This is a frontend-only change with no backend schema or generated API type changes expected.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[lstein]

Code clean. Works as advertised.

I added guards to protect against the negativePromptChanged dispatching when the active model doesn't support a negative prompt. This is to avoid the negative prompt persisting but not visible in the UI and then resurfacing when switching to a model that does support negative prompts.