Redirect writing errors to stdio to stderr

[conniey]

What does this PR do?

This redirects writing exceptions to stdio and writes to err instead. Anything written to stdio is considered a response from the MCP server and is interpreted as JSON RPC. Consequently, is failed to be interpreted by Docker gateway.

In Visual Studio Code, it outputs a warning but keeps reading from stdio which succeeds later on.

2026-05-01 12:09:18.285 [warning] Failed to parse message: "Failed to detect X display. Running in headless mode. Exception type: System.Exception\n"
2026-05-01 12:23:03.878 [debug] [server -> editor] {"result":{"content":[{"type":"text","text":"{\"status\":200,\"message\":\"Success\",\"results\":{\"subscriptions\":[{\"subscriptionId\" ...

In Docker Gateway, it stops reading input from stdio:

Error: MPC 0: calling "tools/call": invalid character 'F' looking for beginning of value

GitHub issue number?

[Link to the GitHub issue this PR addresses]

Pre-merge Checklist

• Required for All PRs
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[Copilot]

Pull request overview

This PR prevents diagnostic output from being written to standard output (stdout), which is reserved for JSON-RPC responses in the MCP server, by redirecting X11 display detection messages to standard error (stderr).

Changes:

• Updated Linux X11 display detection logging in WindowHandleProvider to write to Console.Error instead of Console.WriteLine.
• Simplified the failure-path console output when X11 detection throws.
• Added an Azure.Mcp.Server changelog entry documenting the bug fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
core/Microsoft.Mcp.Core/src/Services/Azure/Authentication/WindowHandleProvider.cs	Redirects X11 detection output from stdout to stderr to avoid JSON-RPC parsing issues.
servers/Azure.Mcp.Server/changelog-entries/1777662676190.yaml	Documents the bug fix in the server changelog entry system.

[jongio]

Clean fix - redirecting X11 display detection diagnostics from stdout to stderr prevents JSON-RPC parsing failures on the stdio transport.

Verified no other Console.WriteLine calls in the server's runtime path need this treatment - the remaining ones in CommandFactory.cs are intentional JSON-RPC output, and CustomHelpAction.cs only runs for --help.

Removing Console.ForegroundColor/Console.ResetColor is also a good call - these set global console state that could leak color codes into stdout on concurrent writes.