fix: correct sbom-create-commands []interface{} fallback and test compilation

[kbarnold]

Summary

• Fixes a logic bug in GetModuleSBomGenCommands introduced by Fix sbom-create-commands type assertion to handle []interface{} #1212: the ok variable from the []string type assertion was never updated after the []interface{} conversion loop, so custom SBOM commands provided as []interface{} (the normal YAML unmarshal result) always fell back to default generation instead of using the provided commands
• Fixes the compile error on master (undefined: getTestPath in internal/commands/commands_test.go) also introduced by Fix sbom-create-commands type assertion to handle []interface{} #1212
• Corrects test assertions to match CmdConverter's actual output structure (working directory prepended as commands[0][0])

Root cause

// ok is false here ([]string cast failed)
customSbomGenCmds, ok := module.BuildParams["sbom-create-commands"].([]string)
if !ok {
    // fills customSbomGenCmds from []interface{}...
}
// ok is STILL false — so this condition is always true, always falling back to default
if !ok || (ok && len(customSbomGenCmds) == 0) {

Fixed by using len(customSbomGenCmds) == 0 which correctly handles all cases.

Test plan

• go build ./internal/commands/... compiles without errors
• go test ./internal/commands/... — all 36 tests pass
• CI on master should go green