Users/axsuarez/cherrypick proactive and docs

[axelsrz]

This pull request makes several improvements to type annotations and documentation throughout the Microsoft Agents Activity and Hosting Core libraries. The main changes include updating optional type hints for model fields, improving docstring formatting and references, and adding precise return type annotations for async methods in the ActivityHandler class.

Type annotation and model improvements:

• Updated fields in ChannelAccount and ConversationAccount models to use Optional type hints, improving clarity for nullable fields. [1] [2] [3]
• Changed the is_group field in ConversationAccount to Optional[bool], and other fields to Optional[NonEmptyString] for better accuracy.

Documentation enhancements:

• Improved docstring formatting by wrapping example values in double backticks for id fields in several models (ChannelAccount, ConversationAccount, TeamsChannelAccount). [1] [2] [3]
• Updated references in docstrings to use fully qualified class names and improved Sphinx-style references for clarity. [1] [2]

Async method return type annotations:

• Added explicit Awaitable[None] return type annotations to all async methods in ActivityHandler for better type safety and documentation. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

General docstring and naming corrections:

• Replaced .. remarks:: with .. note:: in docstrings for consistency and readability. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
• Fixed typos and improved method references in documentation, such as correcting method names for reaction handling.

These changes improve code clarity, maintainability, and developer experience by making type usage and documentation more precise and consistent.