fix(license): use cause-agnostic wording for UNKNOWN category reason

UNKNOWN category can result from categorization issues, not just
non-standard SPDX identifiers. Use "License category could not be
determined" instead of "License not recognized as a standard SPDX
identifier" per reviewer feedback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: a-oren

src/main/java/io/github/guacsec/trustifyda/license/LicenseCheck.java

@@ -147,7 +147,7 @@ public static CompletableFuture<LicenseSummary> runLicenseCheck(
                   if (status == Compatibility.INCOMPATIBLE) {
                     String reason =
                         entry.category() == LicenseCategory.UNKNOWN
-                            ? "License not recognized as a standard SPDX identifier."
+                            ? "License category could not be determined."
                                 + " Manual review recommended to verify compatibility."
                             : "Dependency license(s) are incompatible with the project"
                                 + " license.";