RRC/E1AP: Fix SDAP header mapping in E1AP and test callback header handling

This commit fixes two related issues:

1. SDAP Header Mapping Bug (cucp_cuup_handler.c):
   The original code incorrectly mapped SDAP header configuration from
   internal E1AP representation to ASN.1 enum values. The internal
   representation (`bearer_context_sdap_config_t`) uses true=present/false=absent,
   while ASN.1 enum (`E1AP_SDAP_Configuration_t`) uses
   0=present/1=absent. The fix correctly inverts the mapping:
   - Internal true (1) → ASN.1 0 (present)
   - Internal false (0) → ASN.1 1 (absent)

The commit also clarifies internal representation in the stack.

2. Test Overhead Adjustment (nr-cuup-load-test.c):
   The change in (1) led to a failure in nr_cuup_functional_test
   with assertion: "size % 4 == 0" in recv_ng()

   - Changed UL packet overhead from 3 bytes to 2 bytes in sender_thread()
   - Before the SDAP fix, the test incorrectly accounted for 3 bytes:
     * 2 bytes PDCP header (correct)
     * 1 byte SDAP header (incorrectly included)
   - After the SDAP fix, SDAP headers are correctly set to absent, so
     only 2 bytes (PDCP header) are needed

   This ensures the test sends packets with the correct header size,
   matching what CU-UP expects after the SDAP configuration fix.

The load test does not enable SDAP headers, therefore the original 3-byte
overhead was possibly a workaround for the SDAP header bug.
With the bug fixed, the test correctly uses 2-byte overhead (PDCP header).

tests/nr-cuup/nr-cuup-load-test.c

@@ -418,7 +418,9 @@ static void *sender_thread(void *v)
 {
   struct thr_data *d = v;
 
-  size_t oh = d->is_ul ? 3 : 0;
+  // Overhead: PDCP header (2 bytes) for UL packets
+  // SDAP headers are disabled, so only PDCP header is added
+  size_t oh = d->is_ul ? 2 : 0;
   size_t packet_len = d->data_len + oh;
   uint8_t buf[packet_len];
   memset(buf, 0, packet_len);