Python: Support A2A connections without target URL for Foundry A2ATool

[giles17]

Motivation and Context

This change aligns with Azure SDK's pattern for handling A2A connections that may not have a target URL configured (e.g., Custom Keys connection type).
Their RemoteA2A connection type is not available to GA and hence, we need to use Custom keys

Changes:

• Add conditional base_url support when A2A_ENDPOINT env var is set
• Always use project_connection_id for connection management
• Update documentation to explain optional A2A_ENDPOINT usage

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR adds support for A2A connections without a target URL configured in Azure AI projects, specifically to handle "Custom keys" connection types. The change aligns with Azure SDK patterns for handling A2A connections.

Key Changes

• Refactored A2A tool configuration to be built dynamically before agent creation
• Added conditional support for optional A2A_ENDPOINT environment variable
• Updated documentation to explain the optional endpoint configuration