fix(sidecar): wrap new FFI calls in ddtrace_ffi_try, refresh after fork

Addresses review feedback on PR:
- Wrap `ddog_sidecar_session_set_default_service_name` calls in
  `ddtrace_ffi_try` so transport errors surface in the trace log
  instead of being silently dropped.
- Use `DDOG_CHARSLICE_C("")` instead of hand-rolled CharSlice struct
  literal for the user-defined case (matches the rest of sidecar.c).
- Call `ddtrace_sidecar_update_process_tags()` at the end of
  `ddtrace_sidecar_handle_fork` so the child's fresh sidecar session
  re-learns the svc.* source after fork; without this, child
  telemetry/RC/stats payloads would drop the svc.* tag entirely
  until the next external trigger.

Submodule bump picks up the companion stats-payload fix in
DataDog/libdatadog#2053.

Author: Leiyks

ext/sidecar.c

@@ -153,17 +153,19 @@ void ddtrace_sidecar_update_process_tags(void) {
 
     zend_string *dd_service = get_DD_SERVICE();
     if (dd_service && ZSTR_LEN(dd_service) > 0) {
-        ddog_sidecar_session_set_default_service_name(&DDTRACE_G(sidecar),
-            (ddog_CharSlice){ .ptr = "", .len = 0 });
+        ddtrace_ffi_try("Failed updating sidecar default service name",
+            ddog_sidecar_session_set_default_service_name(&DDTRACE_G(sidecar),
+                DDOG_CHARSLICE_C("")));
     } else {
         zend_string *default_svc = ddtrace_default_service_name();
         if (default_svc) {
             const char *normalized = ddog_normalize_process_tag_value((ddog_CharSlice){
                 .ptr = ZSTR_VAL(default_svc), .len = ZSTR_LEN(default_svc)
             });
             if (normalized) {
-                ddog_sidecar_session_set_default_service_name(&DDTRACE_G(sidecar),
-                    (ddog_CharSlice){ .ptr = normalized, .len = strlen(normalized) });
+                ddtrace_ffi_try("Failed updating sidecar default service name",
+                    ddog_sidecar_session_set_default_service_name(&DDTRACE_G(sidecar),
+                        (ddog_CharSlice){ .ptr = normalized, .len = strlen(normalized) }));
                 ddog_free_normalized_tag_value(normalized);
             }
             zend_string_release(default_svc);
@@ -518,6 +520,10 @@ void ddtrace_sidecar_handle_fork(void) {
 
     if (DDTRACE_G(sidecar)) {
         ddtrace_sidecar_for_signal = DDTRACE_G(sidecar);
+        // The fresh post-fork sidecar session has no record of the svc.*
+        // source. Re-push it so telemetry/RC payloads from the child still
+        // carry svc.user/svc.auto.
+        ddtrace_sidecar_update_process_tags();
     }
 #endif
 }