fix: On Windows, preserve/proxy the exit code of tools, to match behavior on Unix

[cspotcode]

mise x -- node -e "process.exit(123)" should return exit code 123, not 1, and it should not log anything to either stdout nor stderr.

This works as expected on Unix thanks to execvp, but not Windows where the child's exit code is converted to an eyre error which is logged and triggers exit code 1.

This PR fixes Windows to behave like Unix.

Discord context: https://discord.com/channels/1066429325269794907/1314301006992900117/1440123300230729882
Discussion: #6995

Example of the desirable behavior on Linux today. This is also the behavior on Windows after this PR:

$ mise x -- node -e 'process.exit(123)' ; echo "$?"
123
# works the same when calling a shim executable from `mise activate --shims`
$ node -e 'process.exit(123)' ; echo "$?"
123

Bad Windows behavior today, before this PR:

$ mise x -- node -e 'process.exit(123)' ; echo "$?"
mise ERROR command failed: exit code 123
mise ERROR Run with --verbose or MISE_VERBOSE=1 for more information
1

---

Note

On Windows, exec_program now exits with the child process's status code instead of converting non-zero exits into errors.

• CLI runtime (Windows):
  • Update exec_program in src/cli/exec.rs to call std::process::exit(code) on Some(code) from the child, preserving/proxying the child's exit code.
  • Removes eyre error wrapping for non-zero exits; only signal-termination still returns an error.

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