Added iterator agent for ADK pipeline

[vivek100]

Add ArrayIteratorAgent for Array Iteration Functionality

Summary
Implements ArrayIteratorAgent to provide built-in array iteration functionality for the ADK. This agent allows developers to iterate over arrays in session state and pass each element to a sub-agent for processing, addressing a common workflow pattern that was previously missing from the ADK.

Changes Made

• ✅ Added ArrayIteratorAgent class in src/google/adk/agents/array_iterator_agent.py
• ✅ Implemented single sub-agent constraint with validation
• ✅ Added support for nested array key access using dot notation (e.g., user_data.customers)
• ✅ Implemented automatic result collection from sub-agent's output_key
• ✅ Added comprehensive unit tests in tests/unittests/agents/test_array_iterator_agent.py
• ✅ Updated agent exports and imports for proper integration
• ✅ Added documentation and examples

Key Features

1. Single Sub-Agent Pattern: Enforces exactly one sub-agent to keep iteration focused and simple
2. Nested State Access: Supports dot notation for accessing nested arrays (app:config.items, user.data.list)
3. Automatic Collection: Collects results from sub-agent's output_key into an array
4. Configurable Context: Passes each array element via configurable item_key (default: current_item)
5. Optional Limits: Supports max_items parameter for processing subsets
6. Robust Error Handling: Validates array existence, sub-agent count, and handles edge cases

Usage Example

# Sub-agent processes individual items
processor = LlmAgent(
    name="item_processor", 
    instruction="Process: {current_item}",
    output_key="result"
)

# Array iterator
iterator = ArrayIteratorAgent(
    name="batch_processor",
    array_key="items",
    output_key="all_results", 
    sub_agents=[processor]
)

Testing

• ✅ Unit tests cover all core functionality
• ✅ Edge cases tested (empty arrays, missing keys, validation errors)
• ✅ Integration with existing ADK patterns verified
• ✅ Nested state access thoroughly tested
• ✅ Error handling and validation confirmed

Breaking Changes
None - this is a new feature addition that doesn't modify existing functionality.

Documentation

• Added comprehensive docstrings following Google style
• Included usage examples in class documentation
• Added type hints for all parameters and methods

Fixes #1610