.NET: fix: Duplicate CallIds cause Handoff Message Filtering to fail

[lokitoth]

Motivation and Context

Some providers, e.g. Gemini, do not use the CallId mechanism to disambiguate simultaneous function calls. This can result in message lists containing multiple turn to fail to filter properly.

Description

The fix is to take advantage of the expectation that Handoff Orchestration is a "single-speaker" flow, which only has a single active AIAgent per "turn" and an agent's turn is not finished until all outstanding function calls are finished.

This allows us to expect that any ambiguous-CallId FunctionCallContent are either in separate turns or will have had a response before the next issued call with the same Id.

• rebuild message filter to properly handle duplicate CallIds
• add tests
• fixes .NET: [Bug]: HandoffMessagesFilter.FilterMessages NullReferenceException with non-unique CallIds (Gemini/Vertex AI) #5244

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 addresses a .NET handoff-orchestration failure when providers emit non-unique (or missing) tool CallIds (e.g., Gemini via adapters), by reworking how handoff/tool-call messages are filtered before sending history to the next agent.

Changes:

• Rebuild HandoffMessagesFilter to avoid relying on unique CallIds and retroactive removals.
• Add unit tests intended to validate filtering behavior across None, HandoffOnly, and All.
• Add an internal helper for producing the handoff “Transferred.” FunctionResultContent.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffMessagesFilter.cs	Replaces dictionary-based matching with a streaming/ordering-based filter and adds a duplicate-CallId check.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/HandoffMessageFilterTests.cs	Adds coverage for filtering behaviors and CallId uniqueness permutations.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs	Adds an internal helper to create the handoff function result content.