feat: add new model definitions for Google, OpenAI, and xAI

[Kamilbenkirane]

Summary

Adds new model registrations:

• Google: gemini-3.1-flash-image-preview (images), gemini-3.1-flash-lite-preview (text)
• OpenAI: gpt-5.4, gpt-5.4-pro, gpt-5.4-mini, gpt-5.4-nano
• xAI: grok-4.20-beta-0309-reasoning, grok-4.20-beta-0309-non-reasoning

[claude]

Code review

1 issue found.

Bug: Placeholder display_name in gemini-3.1-flash-image-preview

src/celeste/modalities/images/providers/google/models.py, line 105:

display_name="Nano Banana 2",

This appears to be a test/placeholder value that was accidentally left in. Every other model in this file and in this PR uses a descriptive name matching the model ID (e.g., "Gemini 3.1 Flash Lite Preview", "GPT-5.4", "Grok 4.20 Beta 0309 Reasoning").

See:

celeste-python/src/celeste/modalities/images/providers/google/models.py

Lines 103 to 107 in 6a81630

	id="gemini-3.1-flash-image-preview",
	provider=Provider.GOOGLE,
	display_name="Nano Banana 2",
	operations={Modality.IMAGES: {Operation.GENERATE, Operation.EDIT}},
	parameter_constraints={

Suggested fix: Change to display_name="Gemini 3.1 Flash Image Preview".

---

🤖 Generated with Claude Code

[claude]

Code review - 1 issue found: placeholder display_name='Nano Banana 2' in src/celeste/modalities/images/providers/google/models.py line 105 for model gemini-3.1-flash-image-preview. All other models use real product names. Should be something like 'Gemini 3.1 Flash Image Preview'. See:

celeste-python/src/celeste/modalities/images/providers/google/models.py

Lines 103 to 107 in 6a81630

	id="gemini-3.1-flash-image-preview",
	provider=Provider.GOOGLE,
	display_name="Nano Banana 2",
	operations={Modality.IMAGES: {Operation.GENERATE, Operation.EDIT}},
	parameter_constraints={