fix: preserve Antigravity identity on model catalog requests

[mvanhorn]

Antigravity exposes Gemini 3.7 Flash when connected directly, but the model disappears from its selector when 9router MITM is enabled. Adding the Gemini 3.7 tiers to the dashboard mapping catalog in PR #3382 did not fix the native Antigravity catalog returned through the proxy. The MITM passthrough currently rewrites every Antigravity request to the hardcoded IDE version 1.23.2, including non-chat model-catalog calls from newer Antigravity releases. This gives the issue a concrete local cause consistent with the reporter's reproduction on Antigravity 2.11.0.

Summary

Restrict the legacy Antigravity IDE-version override in src/mitm/antigravityIdeVersion.js to generation endpoints that need the existing upstream compatibility workaround. Pass the request URL from src/mitm/server.js into that decision so model-catalog and other non-chat passthrough requests retain their original User-Agent, metadata, body bytes, and content length. Keep the existing chat-request rewrite behavior intact to avoid reopening the compatibility failure the override was introduced to handle.

Tests

• A fetchAvailableModels or equivalent non-generation request with an Antigravity 2.11.0 identity is returned unchanged, preserving both the original User-Agent and metadata.ideVersion so current models remain eligible upstream.
• A generateContent and a streamGenerateContent request still applies the known-good legacy version override and returns the rewritten body for forwarding.
• A non-chat request with malformed or non-JSON content remains byte-for-byte unchanged and does not acquire a rewritten identity.
• A request without an Antigravity User-Agent or metadata identity remains unchanged instead of synthesizing identity fields.

Fixes #3414

[decolua]

Thanks @mvanhorn for the contribution! Reviewed and merged into master. 🙏