fix(apiserver): reject a malformed rusage[] at job submit (bug-1744)

Gitea #47: `xcecli jobs submit -R "rusage[mem=100000000a0]"` was accepted
with a normal "Job <...> is submitted" reply. The apiserver only
length-checked resreq; the real PEG grammar ran later, asynchronously, in
the scheduler's core_build_job(), which logged a server-side WARN and
dropped the job -- it then sat PEND forever with no sign of why.

- libxce: xce_check_rusage_syntax(resreq, &errmsg) runs parse_rusage() on
  every rusage[...] clause (extracted the way section.peg does: up to the
  first ']'), so primary, compound and alternative requests are all
  covered. No resource manager needed, so select[]/order[] resource names
  are still only checked by the scheduler (the documented scope).
- apiserver/backend: ValidateResreqSyntax() cgo wrapper (resreq_check.go);
  the !cgo stub returns nil (no job can be submitted without CGO anyway).
- controllers/jobs.go preCheckJobRequest(): calls it after the length
  check via a package var, so a malformed request is refused up front.

Tests: lib test_check_rusage_syntax (RED before: the reported input was
accepted) and backend TestValidateResreqSyntax (RED before) through the
real grammar; controllers TestPreCheckJobRequest_RejectsMalformedResreq
checks the call site with an injected validator (written with the call
site, so no separate RED). libxce, apiserver backend (cgo) and controllers
tests pass. apiserver/middleware TestLoadJobs fails here and on the
baseline alike: it needs a live apiserver on :8080.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/lib/testsuite/test_resreq.c

@@ -1,3 +1,4 @@
+#include <errno.h>
 #include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -481,11 +482,49 @@ void test_parse_section_span_not_in_select(void)
     FREEUP(errmsg);
 }
 
+/* bug-1744: the apiserver's pre-submit check. Valid rusage clauses (and
+ * requests with none) pass; a malformed one -- the reported
+ * "rusage[mem=100000000a0]" -- is rejected with a message. */
+void test_check_rusage_syntax(void)
+{
+    const char* good[] = {
+        "rusage[mem=100]",
+        "select[type==any] rusage[mem=100, swp=2]",
+        "select[type==any] order[r15m]",
+        "",
+        "{rusage[mem=1]} || {rusage[mem=2]}",
+    };
+    const char* bad[] = {
+        "rusage[mem=100000000a0]",
+        "select[type==any] rusage[mem=]",
+        "{rusage[mem=1]} || {rusage[mem=x]}",
+        "rusage[mem=1",
+    };
+    char*  errmsg;
+    size_t i;
+
+    for (i = 0; i < sizeof(good) / sizeof(good[0]); i++) {
+        errmsg = NULL;
+        TEST_ASSERT_EQUAL_INT_MESSAGE(1, xce_check_rusage_syntax(good[i], &errmsg),
+                                      good[i]);
+        TEST_ASSERT_NULL(errmsg);
+    }
+    for (i = 0; i < sizeof(bad) / sizeof(bad[0]); i++) {
+        errmsg = NULL;
+        TEST_ASSERT_EQUAL_INT_MESSAGE(-EINVAL,
+                                      xce_check_rusage_syntax(bad[i], &errmsg),
+                                      bad[i]);
+        TEST_ASSERT_NOT_NULL_MESSAGE(errmsg, bad[i]);
+        free(errmsg);
+    }
+}
+
 void test_resreq(void)
 {
     printf("\n>> resreq test\n\n");
 
     RUN_TEST(test_parse_section1);
+    RUN_TEST(test_check_rusage_syntax);
     RUN_TEST(test_parse_section2);
     RUN_TEST(test_parse_section_fail);
     RUN_TEST(test_parse_section3);