Add test case for multi-command cancellation behavior

[WesleyJohnson0423]

Description

This PR adds a new test case to verify the Agent's cancellation behavior when processing multiple commands in sequence. Currently, the test suite covers cancellation behavior for single commands, but lacks coverage for scenarios where multiple commands are queued.

The new test case ensures that:

1. When cancellation occurs during the execution of the first command, no function output is emitted
2. Subsequent commands in the sequence are not executed at all after cancellation
3. The execution count of handleExecCommand is exactly 1 (only the first command)

Motivation and Context

Ensuring proper cancellation behavior is critical for the agent's reliability. When a user cancels an agent operation, we need to guarantee that:

• No partially-processed output is shown to the user
• All queued commands are immediately abandoned
• Resources are properly cleaned up

This test strengthens our confidence in the cancellation mechanism by testing a more complex scenario with multiple sequential commands.

Implementation Details

The test creates a custom FakeStream implementation that yields multiple function_call items in sequence, simulating an agent that wants to run multiple shell commands. It then:

1. Starts the agent loop
2. Waits briefly for the first command to begin processing
3. Triggers cancellation before the first command completes
4. Verifies the execution was properly halted and no output was emitted

How Has This Been Tested?

The test has been verified to pass locally and correctly fail when the cancellation behavior is broken.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Test coverage improvement
• Documentation update
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING guidelines
• I have added tests to cover my changes
• All new and existing tests passed

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[WesleyJohnson0423]

I have read the CLA Document and I hereby sign the CLA