.NET: Fix InvokeMcpTool approval path for declarative workflows

[peibekwe]

Motivation and Context

Fix InvokeMcpTool approval path for declarative workflows to remove passing headers.

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.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 91% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by peibekwe's agents

[Copilot]

Pull request overview

This PR fixes the declarative InvokeMcpTool approval flow so transport headers are not exposed through externally surfaced approval requests while preserving header forwarding for actual MCP transport calls.

Changes:

• Removes transport headers from McpServerToolCallContent.AdditionalProperties in approval requests.
• Adds tests validating approval payloads exclude secret headers.
• Adds a test validating direct MCP invocation still forwards headers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeMcpToolExecutor.cs	Stops copying transport headers into approval request tool-call metadata.
dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/InvokeMcpToolExecutorTest.cs	Adds regression coverage for header exclusion from approval content and direct invocation header forwarding.

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 93% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Design Approach

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Automated review by peibekwe's agents