Update to match Google ADK 1.3 patterns

[andrewginns]

This PR updates the repository to align with Google Agent Development Kit (ADK) 1.3 patterns, adds improved support for the ADK web UI, modernises ADK MCP toolset management, updates agent/model references, and improves documentation and usability throughout the project.

Problem:

• The codebase and documentation referenced older ADK versions and patterns.
• The repository lacked first-class support for the ADK web UI and new ADK 1.3 features.
• ADK MCP Toolset management, resource cleanup, and agent initialisation required for better reliability and extensibility.
• Various scripts and documentation were out of sync with recent model releases and best practices.

Solution:

• Refactored agents to use ADK 1.3 patterns, including new toolset/resource management.
• Added explicit support for the ADK web UI (e.g., agent exports, Makefile targets).
• Updated all relevant examples, scripts, and documentation to reference the latest ADK and model versions.
• Improved session, resource, and cleanup handling to prevent errors and ensure robustness.
• Enhanced Makefile and README instructions for reproducibility and clarity.

Unlocks:

• Enables full use of the ADK web UI for agent interaction and visualisation.
• Allows easier migration to future ADK versions and toolset extensions.
• Simplifies onboarding for new users with improved documentation and setup steps.
• Reduces maintenance burden and runtime errors with modern patterns.

Detailed breakdown of changes:

• Makefile: Added new targets for launching the ADK web UI for both basic and multi-agent scenarios.
• README.md and Sub-READMEs: Updated to document new ADK 1.3 usage, model versions, and added instructions for running agents via the ADK web UI.
• agents_mcp_usage/basic_mcp/basic_mcp_use/adk_mcp.py: Major refactor to use new ADK agent initialisation, toolset management, and web UI support, including robust async cleanup.
• agents_mcp_usage/basic_mcp/basic_mcp_use/init.py: Exports the root agent for ADK web UI discovery.
• agents_mcp_usage/basic_mcp/basic_mcp_use/langgraph_mcp.py, oai-agent_mcp.py, pydantic_mcp.py: Updated for new model versions and toolset initialisation via utility functions.
• agents_mcp_usage/multi_mcp/README.md and eval_multi_mcp/README.md: Updated all command references, model arguments, and usage examples to match new patterns.
• Updated all model references to use the latest gemini-2.5-pro-preview-06-05, improved parameterisation, and added missing documentation for new features.
• Improved tool/resource cleanup logic across all relevant scripts.
• (Many incremental improvements and bugfixes not exhaustively listed here.)