feat(session): add LLM API call duration logging

[shoaibansari5398]

What does this PR do?

Fixes #9037

Adds duration logging for LLM API calls to help users monitor and analyze API latency.

How did you verify your code works?

Changes

• Modified packages/opencode/src/session/llm.ts.
• Capture startTime before calling streamText().
• Wrap fullStream async iterable to log duration on stream completion.
• Log format: service=llm ... stream completed ... duration=xxx