.NET: Dispose AI context streaming enumerators on early exit

[quick_fox (quifox)]

Motivation & Context

The AI context streaming decorators manually own inner async enumerators. When an outer consumer stops early, those enumerators are not disposed, so iterator cleanup and owned resources may not be released.

Description & Review Guide

• What are the major changes? Both streaming decorators now establish async disposal ownership immediately after acquiring the inner enumerator, with regression coverage for early consumer exit.
• What is the impact of these changes? Inner iterator cleanup now runs on early exit while preserving normal streaming output, exception, and cancellation behavior.
• What do you want reviewers to focus on? The change only affects enumerator lifetime management after successful acquisition.

Related Issue

Fixes #8810

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

🟢 Approval recommended

The narrowly scoped lifetime fix is correct and covered by targeted regression tests.

Review effort: Balanced
Findings: None

What changed in this PR

Ensures .NET AI context streaming decorators dispose inner enumerators when consumers exit early.

Changes:

• Adds asynchronous disposal ownership to both streaming decorators.
• Adds regression tests covering early consumer exit.

File	Description
MessageAIContextProviderAgent.cs	Disposes the inner agent enumerator.
AIContextProviderChatClient.cs	Disposes the inner chat enumerator.
MessageAIContextProviderAgentTests.cs	Tests early-exit disposal.
AIContextProviderChatClientTests.cs	Tests early-exit disposal.

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