fix: stop hanging project typechecks

[christopher-buss]

Description

Closes #9494 - vitest run hanging after successful project typechecking and leaving an orphaned tsc or tsgo --watch process.

Project configs previously resolved watch from configDefaults instead of inheriting the root configuration. In an interactive terminal, this produced watch: false at the root but watch: true in the project, causing the typechecker to start in watch mode during vitest run.

This PR:

• propagates the root watch value to project configs
• destroys the typechecker output streams during shutdown
• terminates the complete process tree on Windows
• uses a detached process group for tree termination on other platforms
• adds regression coverage for project configuration and process cleanup

The terminal-specific behavior was also verified using a forced interactive-terminal reproduction. After the fix, both root and project resolve watch: false, Vitest exits normally, and no watched typechecker process remains.

AI-assisted by Codex and reviewed using Claude Opus + a human (myself)!

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[sheremet-va]

Can you explain in the comment why do we need this branching?

If the issue is with tinyexec, can we use child_process directly? Or should the fix be in tinyexec?

[christopher-buss]

I've opened a feature request for this on tinyexec to see if they're interested. Execa has just implemented a version of this (despite an issue for this being open for 6 years) - but they may accept a contribution. Node doesn't offer this natively, so it's less an "issue" I'd say and more an API that doesn't exist. Depends if you'd want to wait for something in the library here (that may / may not be accepted or not)! I've pushed a change for now to make the code a bit easier to read, let me know if you want me to draft this PR in the meantime while we wait to see if tinyexec will support this.

[christopher-buss]

FWIW execa https://github.com/sindresorhus/execa/pull/1256/changes#diff-5af77adef0297f94becc96dd37c6dcf70dc340b32b2657dd167b215db6e95e15 also does the branching due to the different process handling by each OS