fix: align session and message API payloads with OpenCode schema

[AhmedB479]

Problem

Two API payload mismatches were causing 400 BadRequest errors when communicating with the OpenCode server.

Fix 1 — Session creation (POST /session)
The session endpoint expects id for the model identifier, but the node was sending modelID:

// Before (broken)
{ "model": { "providerID": "anthropic", "modelID": "claude-3-5-sonnet-20241022" } }

// After (fixed)
{ "model": { "providerID": "anthropic", "id": "claude-3-5-sonnet-20241022" } }

Fix 2 — Message sending (POST /session/{id}/message)
The message endpoint does not accept a model field — the session already knows its model. Sending it caused a 400 rejection:

// Before (broken)
{ "parts": [...], "model": { ... }, "agent": "chat" }

// After (fixed)
{ "parts": [...], "agent": "chat" }

Bonus fixes

• Added jest.config.js with ts-jest preset so the test suite can actually run (it was broken out of the box due to missing Jest config)
• Added !jest.config.js exception to .gitignore since *.js was swallowing it
• Updated the session creation test assertion to expect the corrected id field

Test plan

• npm run type-check passes
• npm test passes