Update deprecated @session decorator references to @trace in v2 documentation

## Problem
The v2 documentation still contains references to the deprecated @session decorator, which should be replaced with @trace.

## Files to Update
- docs/v2/concepts/decorators.mdx
- docs/v2/concepts/spans.mdx
- docs/v2/usage/tracking-agents.mdx

## Changes Needed
- Update decorator table to show @trace instead of @session
- Update all code examples to use @trace
- Update text descriptions to reference trace instead of session
- Ensure migration guidance is clear

## Context
The @session decorator is deprecated but maintained for backward compatibility. New documentation should consistently use @trace as the recommended approach.

## Assignee
Please assign to Dwij when creating