LangGraph — Align with gen_ai RFC #153

Align the LangGraph integration with [RFC #153]().

### Attribute renames

- [ ] `gen_ai.system` → `gen_ai.provider.name`

### Required attributes

- [ ] `gen_ai.operation.name` set on all spans
- [ ] `gen_ai.provider.name` set on all AI Client spans

### Span changes

- [ ] Remove `gen_ai.create_agent` span type
- [ ] Ensure proper span hierarchy: `invoke_agent` → AI Client → `execute_tool` → AI Client

### Message format

- [ ] `content` → `parts` in `gen_ai.input.messages` and `gen_ai.output.messages`