feat: Add documentation for early exit mechanism to SequentialAgent using escalate action #2999

[tommyhutcheson]

Summary

Adds documentation for the early exit mechanism in SequentialAgent using the exit_sequence tool.

Related Python PR: google/adk-python#2999

Changes

New Section: "Early Exit with exit_sequence"

• Explains when to use early exit (validation failures, empty results, blocking conditions)
• Usage example showing how to call exit_sequence from a tool
• Practical example: Search → Analysis → Report pipeline
• Cross-reference to LoopAgent's exit_loop for consistency

Corrected Implementation Details:

• Clarified that exit_sequence is supported in both async and live streaming modes
• Documented that escalate action provides immediate termination in live mode
• Noted that task_completed() remains available for natural completion signals

Documentation Structure

The new section is placed between "How it works" and "Full Example: Code Development Pipeline" to maintain logical flow:

1. Introduction and use cases
2. Basic usage with code example
3. Practical pipeline example
4. Live mode support note
5. Comparison to LoopAgent

Key Updates

• Added import examples showing exit_sequence alongside other tools
• Included practical search pipeline demonstrating early exit on empty results
• Cross-referenced LoopAgent documentation for consistent patterns
• Clarified behaviour in both async and live modes

Fixes documentation gap for feature introduced in google/adk-python#2999