.NET: Propagate ChatHistoryMemoryProvider caller cancellation

[quick_fox (quifox)]

Motivation & Context

ChatHistoryMemoryProvider uses best-effort handling for vector search and history storage failures, but its generic exception handlers also swallow caller-triggered cancellation. This can allow search or persistence work to continue after the caller has cancelled the operation.

Description & Review Guide

• What are the major changes? The search and storage catch filters now propagate OperationCanceledException when the caller token is actually cancelled, with regression coverage for both caller-side and provider-side cancellation.
• What is the impact of these changes? Caller cancellation is preserved while unrelated provider failures and provider-side cancellation continue to use the existing best-effort behavior.
• What do you want reviewers to focus on? The filter checks the caller token so the change does not broaden failure propagation beyond caller-triggered cancellation.

Related Issue

Fixes #8811

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 title prefix.

[Copilot]

Copilot review overview

🟡 Changes recommended

The storage cancellation test lacks an UpsertAsync setup and therefore fails without exercising the intended behavior.

Review effort: Balanced
Findings: 1

Open (1)

• Configure UpsertAsync mock to exercise cancellation filtering · New

What changed in this PR

Preserves caller cancellation while retaining best-effort handling for provider-side failures.

Changes:

• Filters search and storage exceptions based on caller cancellation.
• Adds cancellation regression tests.

File	Description
ChatHistoryMemoryProvider.cs	Propagates caller-triggered cancellation.
ChatHistoryMemoryProviderTests.cs	Adds search and storage cancellation tests.

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