fix: guard non-string JSON type fields before strcmp in C API driver

[ramakrishnap-nv]

Summary

parse_cuopt_json() in benchmark_apis/c_api_driver/cuopt_json_to_c_api.c read JSON array
elements via cJSON's valuestring and passed them straight to strcmp(). cJSON sets
valuestring to NULL for non-string nodes (numbers, bools, null, arrays, objects), so a
non-string element produced strcmp(NULL, ...) — a NULL-pointer dereference that crashes the
driver (SIGSEGV; UBSan nonnull violation) on untrusted cuOpt-JSON input.

Two sinks were affected:

• variable_types[] parsing loop (strcmp(type_str, "I"))
• constraint_bounds.types[] fallback (strcmp(type, "L"|"G"|"E"))

Fix

Guard both sinks with cJSON_IsString() + a NULL check before each strcmp, falling back to
the existing default (CONTINUOUS variable / unconstrained row) for non-string entries. This
matches the cJSON_IsString(...) ? ... : NULL pattern already used in parse_numeric_value().

Reproducer

{
  "csr_constraint_matrix": {"offsets": [0, 1], "indices": [0], "values": [1.0]},
  "objective_data": {"coefficients": [1.0], "offset": 0.0},
  "variable_bounds": {"lower_bounds": [0.0], "upper_bounds": [0.0]},
  "maximize": false,
  "variable_types": [123]
}

A non-string constraint_bounds.types element (e.g. "types": [123]) triggers the same crash
on the G/E branches.

Verification

Built the translation unit under ASan + UBSan (real parser, stubbed solver). Both a
number-valued variable_types element and a number-valued constraint_bounds.types element
crashed with a SEGV in strcmp before the fix and parse cleanly after.

Found via fuzzing; robustness / availability hardening (not a production solver path).

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