Add GPT 5.5

[devin-ai-integration]

Summary

Add GPT 5.5 as a model option in both the app client and server.

• Added gpt55 entry to MODELS in app/constants.ts with the OpenAI icon
• Added gpt55: 'gpt-5.5' to the server model map in server/src/chat/gpt.ts

Review & Testing Checklist for Human

• Verify GPT 5.5 appears in the model selector in the app
• Test sending a message using GPT 5.5 to confirm server routing works

Notes

Follows the same pattern used for existing GPT models (GPT 5.2, GPT 5 Mini).

Link to Devin session: https://app.devin.ai/sessions/7c240e12b92c42b5951385e57935e159
Requested by: @dabit3

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Devin Review

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.