refactor(command): remove the relative PATH workaround after the which-rs release

[RSS1102]

Follow-up to #2350. This does not block the current fix.

crates/vp_command/src/lib.rs currently searches PATH entries individually because the released version of which_in does not resolve ordinary relative PATH entries against its provided cwd.

Upstream fix: harryfei/which-rs#128

Follow-up work

Once the upstream fix is merged and released:

• update the workspace which dependency in Cargo.toml and regenerate Cargo.lock
• remove is_plain_relative_path and resolve_bin_from_path_entry from crates/vp_command/src/lib.rs
• replace the explicit-path branch and per-entry lookup in resolve_bin with one which::which_in(bin_name, Some(path_env), cwd) call
• keep behavior tests for relative entries, lookup order, Unix empty entries, a Unix cwd containing :, explicit relative programs, and the cross-platform CLI snapshot
• remove only the helper-specific Windows classification tests after the helpers are deleted

Related: #2326

[fengmk2]

@RSS1102 https://github.com/harryfei/which-rs/releases/tag/8.0.6 released

[RSS1102]

@RSS1102 https://github.com/harryfei/which-rs/releases/tag/8.0.6 released

Thank you for your awakening, this will be a happy weekend ❤️