refactor(ai): name provider turn operations

[nexxeln]

Summary

• rename the root one-provider-turn facade to LLM.generateTurn and LLM.streamTurn
• remove the ambiguous root generate and stream names while reserving them for future complete runs
• migrate root-facing tests, examples, documentation, and tracing

Testing

• bun test in packages/ai (444 passed, 28 skipped)
• bun typecheck in packages/ai
• bun typecheck in packages/core
• bun run build in packages/ai
• repository-wide bun turbo typecheck --concurrency=3 via the pre-push hook