.NET: Propagate TextSearchProvider caller cancellation - #8796

Motivation & Context

TextSearchProvider treats failures during BeforeAIInvoke search as best-effort and returns no additional context. The generic exception handler also catches caller-triggered cancellation, which can let the agent invocation continue after the caller has cancelled the operation.

Description & Review Guide

• What are the major changes? The search failure catch filter now lets OperationCanceledException propagate when the caller-provided cancellation token is cancelled. A regression test covers the cancellation path.
• What is the impact of these changes? Caller cancellation is preserved while ordinary search failures, including unrelated provider-side failures, keep the existing best-effort behavior.
• What do you want reviewers to focus on? The catch filter intentionally checks the caller token so provider-side OperationCanceledException values remain handled when the caller did not cancel.

Related Issue

Fixes #8795

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. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.