Fix daemon relay exit classification

[dibarbet]

Fixes #84662

When the daemon endpoint is lost, both directional relay tasks can complete against the server endpoint. The thin client previously interpreted any pair of completed relay tasks as a clean shutdown and returned exit code 0.

Classify relay completion using the endpoint reported by each copy direction:

• two server-endpoint completions indicate daemon connection loss
• a single completion indicates loss of that endpoint
• any other pair indicates clean shutdown

A clean editor shutdown may complete both copies at the editor because the editor closes its bidirectional transport after sending LSP exit, before the daemon closes its side.

Validation:

• Built src\LanguageServer\roslyn-language-server\roslyn-language-server.csproj
• Ran the existing daemon lifecycle tests for clean shutdown, daemon loss, editor loss, and keepalive behavior across both transports (7 passed)
• Ran KeepAlive_DaemonReusedWithinWindow_ThenExitsWhenIdle in Release (passed)

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR updates the roslyn-language-server thin client’s daemon relay shutdown logic so it can reliably distinguish a clean LSP shutdown from loss of either the editor or daemon endpoint, avoiding incorrectly returning exit code 0 when the daemon is killed.

Changes:

• Replace the relay “both sides closed” heuristic with an explicit RelayCompletionKind classification.
• Determine clean shutdown vs. editor/daemon connection loss by comparing the endpoints reported by each relay direction.

Show a summary per file

File	Description
src/LanguageServer/roslyn-language-server/Program.cs	Consumes the new relay completion classification to choose the correct exit code (success vs. editor/daemon lost).
src/LanguageServer/roslyn-language-server/LspRelay.cs	Introduces RelayCompletionKind and updates relay completion logic to classify shutdown based on which endpoint each copy direction reports.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/LanguageServer/roslyn-language-server/LspRelay.cs:41

• RelayAsync now awaits completedTask before canceling the other copy direction. If CopyUntilClosedAsync faults unexpectedly, this will throw before cancellationSource.Cancel() runs, potentially leaving the other copy task running and holding resources/streams open. Wrapping the awaits in a try/finally (or deferring the await until after cancel) keeps the previous guarantee that the non-winning direction is always canceled.

        var completedTask = await Task.WhenAny(editorToServer, serverToEditor).ConfigureAwait(false);
        var closedEndpoint = await completedTask.ConfigureAwait(false);

        // Give the other direction a brief window to finish on its own. If both copies terminate at the server,
        // the server connection was lost and caused both directions to stop. Any other pair is a clean shutdown:

• Files reviewed: 2/2 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0 new