feat: add Close() method to Toolset interface

[ju4nv1e1r4]

Description

This PR adds a Close() error method to the tool.Toolset interface to support resource cleanup and graceful shutdown of toolsets.

Motivation:

1. Parity with adk-python: Aligns the Go SDK with the Python SDK (BaseToolset.close), as adk-python is the source of truth.
2. Resource Management: Addresses the need to properly close persistent connections, specifically for the MCP Toolset (related to issue Add Close method to tool.Toolset interface #481 and mcptoolset: cached MCP session not validated, causes "connection closed" errors on subsequent requests #399), preventing resource leaks (e.g., open sessions/connections).

Changes:

• Updated tool.Toolset interface to include Close() error.
• Implemented Close() in filteredToolset to delegate the call to the underlying toolset.
• Implemented Close() in mcptoolset to clean up the client session/transport.
• Updated all internal mocks and test implementations to satisfy the new interface.

Related Issues

Fixes #481
Relates to #399

Breaking Change

Yes.
This modifies the public tool.Toolset interface. Any external implementation of this interface will need to implement the Close() method.

Testing Plan

I have verified the changes using the following methods:

1. Unit Tests:

• Added TestFilteredToolset_Close in tool/tool_test.go to verify that filteredToolset correctly delegates the Close call to the underlying toolset.
• Updated existing tests and mocks to ensure no regressions.

2. Regression Testing:
Ran the full test suite with race detection enabled. All tests passed.

go test -v -race ./...

3. Format & Style:
Verified code compliance with gofmt -s.

[gemini-code-assist]

Summary of Changes

Hello @ju4nv1e1r4, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial Close() method to the tool.Toolset interface, standardizing resource cleanup across toolset implementations. This enhancement ensures better resource management, particularly for persistent connections like those in the MCP Toolset, and aligns the Go SDK with its Python counterpart. The change facilitates graceful shutdown and prevents potential resource leaks, making the system more robust and maintainable.

Highlights

• Interface Extension: The tool.Toolset interface has been extended to include a new Close() error method, enabling explicit resource cleanup for toolset implementations.
• Resource Management: The mcptoolset implementation now includes a Close() method to gracefully shut down and clean up the underlying MCP client session and transport, preventing resource leaks.
• Delegation for Filtered Toolsets: The filteredToolset now implements the Close() method by delegating the call to its wrapped underlying toolset, ensuring proper cleanup through the filter.
• Breaking Change: This change is a breaking change as any external implementations of the tool.Toolset interface will now need to implement the new Close() method.
• Test Coverage and Examples: New unit tests have been added for filteredToolset.Close(), and existing tests and examples (like examples/mcp/main.go and tool/mcptoolset/set_test.go) have been updated to utilize the new Close() method with defer.

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[gemini-code-assist]

Code Review

This pull request introduces a Close() method to the tool.Toolset interface to enable resource cleanup, which is a good addition for managing resources like network connections. The implementation is propagated correctly to filteredToolset and mcptoolset. My main feedback is to consistently handle the error returned by the new Close() method, especially in deferred calls within tests, to prevent potential issues from being silently ignored.

[ju4nv1e1r4]

Another test: