Wait for file change when process exits

[tmat]

Fixes #47425.

There are two code paths that may be executed when process exists ,depending on exact timing of task cancellation.
One of the code paths was incorrectly not waiting for a file update before restarting the app.

[tmat]

@phil-allen-msft ptal

PR Overview

This PR fixes an issue where the file change is not awaited when a process exits. It includes changes to update the file change callback logic in HotReloadDotNetWatcher.cs and updates the ProcessCleanupTimeout configuration in EnvironmentVariables.cs.

• Updated file change callback behavior to wait for a file update on process exit.
• Modified ProcessCleanupTimeout configuration to use a default value of 5 seconds.
• Adjusted the ReadTimeSpan method to accept a default value for improved configurability.

Reviewed Changes

File	Description
src/BuiltInTools/dotnet-watch/HotReloadDotNetWatcher.cs	Updated the catch block to always wait for file change when the process exits.
src/BuiltInTools/dotnet-watch/EnvironmentVariables.cs	Modified ProcessCleanupTimeout to include a configurable default value and updated ReadTimeSpan accordingly.

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/BuiltInTools/dotnet-watch/HotReloadDotNetWatcher.cs:223

• The removed assignment to false followed by setting the value to true in the catch block may cause confusion. Consider removing the commented-out assignment to clarify that the intended behavior is to wait for a file change when the process exits.

waitForFileChangeBeforeRestarting = false;

src/BuiltInTools/dotnet-watch/EnvironmentVariables.cs:51

• The original ReadTimeSpan method signature has been removed in favor of one that requires a default value. Please verify that all calls to ReadTimeSpan have been updated accordingly to prevent any unintended behavior.

private static TimeSpan ReadTimeSpan(string variableName)