Session Fork/Clone API for Conversation Branching

**Is your feature request related to a problem? Please describe.**

When building applications that need to explore multiple conversation paths from a single point, there's no way to create a copy of an existing session without modifying the original.

The current `rewind_async()` API modifies the original session's state and artifacts, making it impossible to:
- Preserve the original conversation while exploring alternative paths
- Create "what-if" branches from a conversation midpoint
- Allow users to fork a conversation and continue in different directions

Example scenario: A user is at turn 5 of a conversation and wants to try a different approach from turn 3, but also keep the original turn 3→5 history intact. Currently this is not possible.

**Describe the solution you'd like**

Add a `fork_session()` or `clone_session()` API that creates a new independent session by copying events, state, and artifact references up to a specified invocation point.

```python
# Option A: In SessionService
new_session = await session_service.fork_session(
app_name=app_name,
user_id=user_id,
source_session_id=original_session.id,
fork_before_invocation_id="inv_3", # Fork point
)

# Option B: In Runner (similar to rewind_async)
new_session = await runner.fork_async(
user_id=user_id,
session_id=session_id,
fork_before_invocation_id="inv_3",
)
```

The original session remains completely unchanged.

**Describe alternatives you've considered**

1. **Rewind then copy**: Doesn't work because `rewind_async()` modifies the original session's state and artifacts before any copy could be made.

2. **Manual event copying**: Requires manually iterating events, recomputing state deltas, and handling artifact references. Error-prone and not officially supported.

3. **External backup before rewind**: Outside ADK's scope, doesn't integrate with artifact versioning, and requires users to implement their own session serialization.

**Additional context**

- Related existing feature: `rewind_async()` in `runners.py:509-647` - similar pattern could be followed
- Use cases: conversation branching UI, A/B testing agent responses, debugging/development snapshots
- I'm willing to contribute this feature if the design is approved