fix: support relative forward-slash paths on Windows

[tianrking]

Summary

• run relative executable paths through standard Windows command-line quoting instead of passing them verbatim to cmd.exe /C
• normalize forward slashes only in that executable path, leaving slash-containing command arguments unchanged
• cover the regression and preserve the existing quoted-command behavior on Windows

Fixes #818

Root cause

The default Windows executor appended the full command line with raw_arg(). For a command beginning with ./ or ../, cmd.exe interpreted the leading . as a command instead of a relative path.

Validation

• cargo fmt -- --check
• cargo clippy --locked --all-targets
• cargo test --locked normalizes_only_the_relative_executable_path
• cargo test --locked windows_runs_executable_paths_with_forward_slashes
• cargo test --locked windows_quote_args

cargo test --locked exercises the new regression and 34 of 36 integration tests on Windows. The remaining two integration tests invoke Unix shell syntax (echo '...' and sleep) without a Windows guard; they are unrelated to this change.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve raw cmd quoting for relative commands

On Windows, this branch switches cmd.exe /C from raw_arg to arg whenever the benchmark starts with ./ or ../. For a command such as ./tool.exe "input file.txt", Rust applies C-runtime escaping to the inner quotes (for example \"...\"), but cmd.exe /c has different escaping rules—the Rust raw_arg docs call out this exact case: https://doc.rust-lang.org/std/os/windows/process/trait.CommandExt.html#tymethod.raw_arg. The result is that quoted arguments after a relative executable can be split or delivered with literal quotes/backslashes; normalize the executable path but keep appending the resulting command string with raw_arg so existing quoted-command behavior is preserved.

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