fix(opencode): prevent Copilot 400 when tool description is empty

[amsminn]

Issue for this PR

Closes #14488

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes Copilot request validation failure when an MCP/OpenAI-compatible tool has an empty description.

Sending tools[].function.description: "" to Copilot Chat Completions returns HTTP 400 Bad Request (reproduced on 2026-02-22), while omitting function.description returns HTTP 200. Because the field is optional in Copilot SDK types, this PR normalizes empty/whitespace descriptions to undefined (field omission) instead of injecting synthetic text like No description. This keeps the original tool meaning and removes the 400 failure:

• Tool.description?: string
  https://github.com/github/copilot-sdk/blob/main/nodejs/src/types.ts#L144
• defineTool(..., { description?: string, ... })
  https://github.com/github/copilot-sdk/blob/main/nodejs/src/types.ts#L158

This PR omits empty/whitespace function.description values and adds two tests.

Changed code:

• packages/opencode/src/provider/sdk/copilot/chat/openai-compatible-prepare-tools.ts:53
• packages/opencode/test/provider/copilot/copilot-chat-model.test.ts:591
• packages/opencode/test/provider/copilot/copilot-chat-model.test.ts:596

How did you verify your code works?

Verification date: 2026-02-22

1. Reproduced issue with Copilot API (claude-sonnet-4.6, stream=true):
   • request with function.description="" -> HTTP 400 (Bad Request)
2. Verified fix behavior:
   • same request with function.description omitted -> HTTP 200 (SSE stream starts)
3. Ran local tests:
   • bun test test/provider/copilot/copilot-chat-model.test.ts (run from packages/opencode)
   • Result: 13 pass, 0 fail
4. Reproduction + validation commands used:

cd <repo-root>
tok=$(gh auth token)

code=$(curl -sS -o /tmp/copilot_empty.txt -w "%{http_code}" https://api.githubcopilot.com/chat/completions \
  -H "Authorization: Bearer $tok" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-sonnet-4.6","messages":[{"role":"user","content":"hi"}],"tools":[{"type":"function","function":{"name":"t","description":"","parameters":{"type":"object","properties":{"x":{"type":"string"}}}}}],"tool_choice":"auto","stream":true}')
echo "empty_description_status=$code"
head -n 3 /tmp/copilot_empty.txt

code=$(curl -sS -o /tmp/copilot_omit.txt -w "%{http_code}" https://api.githubcopilot.com/chat/completions \
  -H "Authorization: Bearer $tok" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-sonnet-4.6","messages":[{"role":"user","content":"hi"}],"tools":[{"type":"function","function":{"name":"t","parameters":{"type":"object","properties":{"x":{"type":"string"}}}}}],"tool_choice":"auto","stream":true}')
echo "omit_description_status=$code"
head -n 3 /tmp/copilot_omit.txt

cd packages/opencode
bun test test/provider/copilot/copilot-chat-model.test.ts

Screenshots / recordings

N/A - no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR