fix(cmd): use exec.LookPath in Which() so detection works without external 'which'

[Sanjays2402]

What this PR does / why we need it?

Fixes #12605.

cmd.Which() previously shelled out to which <name> to determine whether a binary was on PATH. On distributions that do not ship a which package by default — Arch Linux is the canonical example, but the same applies to several minimal container images — which itself is missing from PATH, so every call to Which() returned false. As a result, 1Panel reported core dependencies (notably Docker) as not installed even when they were running normally:

GET /api/v2/containers/docker/status
{"code":200,"message":"","data":{"isActive":false,"isExist":false}}

…on a host where docker.service was active, docker version worked, and /var/run/docker.sock was reachable. The reporter confirmed that simply dropping a stub /usr/local/bin/which script restored detection.

Summary of your change

Switch the primary path to Go's exec.LookPath, which uses the process PATH directly and has no external binary dependency. The original shell-out is preserved as a fallback so any environment where the agent's PATH differs from the user's interactive shell PATH (the reason the shell-out existed in the first place) keeps working unchanged.

Both copies of the helper are updated:

• agent/utils/cmd/cmd.go
• core/utils/cmd/cmd.go

func Which(name string) bool {
    if _, err := exec.LookPath(name); err == nil {
        return true
    }
    // Fallback: original shell-out, preserved for compatibility.
    stdout, err := RunDefaultWithStdoutBashCf("which %s", name)
    if err != nil || (len(strings.ReplaceAll(stdout, "\\n", "")) == 0) {
        return false
    }
    return true
}

A small unit test is added to each package guarding the regression:

• Which("sh") is true (sh is on every Unix-like host the CI runs on, regardless of whether which itself is installed).
• Which("definitely-not-a-real-binary-xyzzy-1panel") is false.

Verification

$ cd agent && go test ./utils/cmd/ -v
=== RUN   TestWhich_ExistingBinary
--- PASS: TestWhich_ExistingBinary
=== RUN   TestWhich_MissingBinary
--- PASS: TestWhich_MissingBinary
PASS

$ cd core && go test ./utils/cmd/ -v
=== RUN   TestWhich_ExistingBinary
--- PASS: TestWhich_ExistingBinary
=== RUN   TestWhich_MissingBinary
--- PASS: TestWhich_MissingBinary
PASS

go vet and gofmt are both clean on the touched files.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed. (No user-facing behaviour or docs change — Which() is internal.)

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