.NET: Keep resilient tool-call snapshots consistent - #9080

Motivation & Context

With resilient background hosting, a non-workflow agent session can be saved after a function_call output item completes but before its matching tool result is appended to the session. If the process exits in that interval, recovery restores an unmatched call and the next Responses request fails with No tool output found for function call. This makes recovery fail during a normal tool-heavy turn.

Description & Review Guide

• What are the major changes? Track emitted function-call IDs until matching function_call_output items complete. Skip incremental and final session saves while any call remains unresolved. Add unit tests for parallel calls with sequential results and for an unanswered call.
• What is the impact of these changes? No public API changes. Resilient non-workflow snapshots wait until calls are paired; workflow checkpoint saves and non-resilient behavior remain unchanged.
• What do you want reviewers to focus on? Verify that the stream item types and call IDs accurately represent completion, especially for parallel calls, and that workflow checkpoint behavior remains unchanged.

Related Issue

Fixes #9033

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.