fix(session): keep AI SDK tool result names

[cyq1017]

Issue for this PR

Closes #31646

Type of change

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

What does this PR do?

When the AI SDK emits a tool-result event after the local tool-call name cache misses, opencode currently labels the result as unknown. The event still carries toolName, so this uses that value before falling back to unknown.

This keeps the tool result paired with the real tool name and avoids feeding an unknown tool result back into the session history.

How did you verify your code works?

• bun test test/session/llm-ai-sdk.test.ts
• bun test test/session/llm.test.ts
• bun test test/session/processor-effect.test.ts
• bun prettier --check packages/opencode/src/session/llm/ai-sdk.ts packages/opencode/test/session/llm-ai-sdk.test.ts
• git diff --check

I also ran the local path lint command; it exited 0 with existing warnings in ai-sdk.ts.

The pre-push hook runs full monorepo typecheck and currently fails in unrelated packages/ui / packages/enterprise files before this change. This PR only changes the AI SDK adapter and its focused test.

Screenshots / recordings

N/A

Checklist

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