fix: disable IDE streaming sink by default to prevent test runner crashes

[thomhurst]

Summary

• Disables IdeStreamingSink by default — it was causing crashes in Rider and VS Code when tests write to Console.WriteLine followed by an async delay
• The root cause is the Microsoft Testing Platform's TestApplicationResult.ConsumeAsync throwing ArgumentException on duplicate TestNodeUid keys, triggered by the sink's output update + heartbeat pattern
• The feature can be re-enabled via the TUNIT_ENABLE_IDE_STREAMING=1 environment variable for users who want real-time output streaming

Closes #4749

Test plan

• Verify tests with Console.WriteLine + Task.Delay no longer crash in Rider/VS Code
• Verify setting TUNIT_ENABLE_IDE_STREAMING=1 re-enables the streaming behavior
• Verify test output is still visible in IDE test explorers after test completion (via ConsoleOutputSink / TestOutputSink)

🤖 Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

This is a clean, well-scoped fix that:

• Correctly addresses the IDE streaming sink crash issue by making it opt-in via environment variable
• Uses appropriate string comparison for environment variable checking (defaults to disabled when not set)
• Maintains all existing functionality with proper guard conditions
• Includes clear documentation updates explaining the change and usage

The approach is sound: disabling a problematic feature by default while allowing users to enable it when needed is the right trade-off for stability.