[#28913] YSQL: Fix dangling std::string::c_str() setting up YbcFlushDebugContext

Summary:
cbbb323 adds a few places where we do something like the following:
```
debug_context.strarg2 = row_id.ybctid().ToDebugHexString().c_str();
```
where `ToDebugHexString()` returns a new `std::string`. The backing `std::string` is destroyed
at the end of the assignment, so `debug_context.strarg2` now contains a dangling pointer. This
is caught in newer clang versions (clang21) with the following error:
```
../../src/yb/yql/pggate/pg_operation_buffer.cc:382:35: error: object backing the pointer 'debug_context.strarg2' will be destroyed at the end of the full-expression [-Werror,-Wdangling-assignment-gsl]
  382 |           debug_context.strarg2 = row_id.ybctid().ToDebugHexString().c_str();
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Fix this by storing the temporary std::string in a separate variable.
Jira: DB-18636

Test Plan: Jenkins. Also compiled locally with clang21 where this fails to compile.

Reviewers: kramanathan, jason

Reviewed By: kramanathan, jason

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D47365

Author: es1024

src/yb/yql/pggate/pg_operation_buffer.cc

@@ -374,12 +374,14 @@ class PgOperationBuffer::Impl {
       if (PREDICT_FALSE(!keys_.insert(row_id).second)) {
         RETURN_NOT_OK(CheckDuplicateInsertForFastPathCopy(table, stmt_type, need_transaction));
 
+        std::string ybctid_hex;
         YbcFlushDebugContext debug_context {};
         debug_context.reason = YB_CONFLICTING_KEY_WRITE;
         debug_context.oidarg = table.pg_table_id().object_oid;
         if (need_flush_context) {
           debug_context.strarg1 = table.table_name().table_name().c_str();
-          debug_context.strarg2 = row_id.ybctid().ToDebugHexString().c_str();
+          ybctid_hex = row_id.ybctid().ToDebugHexString();
+          debug_context.strarg2 = ybctid_hex.c_str();
         }
         RETURN_NOT_OK(Flush(debug_context));
         keys_.insert(row_id);

src/yb/yql/pggate/pg_session.cc

@@ -1324,9 +1324,13 @@ Result<int64_t> PgSession::GetCronLastMinute() { return pg_client_.GetCronLastMi
 
 Status PgSession::AcquireAdvisoryLock(
     const YbcAdvisoryLockId& lock_id, YbcAdvisoryLockMode mode, bool wait, bool session) {
+  std::string lock_id_str;
   YbcFlushDebugContext debug_context {};
   debug_context.reason = YbcFlushReason::YB_ACQUIRE_ADVISORY_LOCK;
-  debug_context.strarg1 = yb_debug_log_docdb_requests ? ToString(lock_id).c_str() : nullptr;
+  if (yb_debug_log_docdb_requests) {
+    lock_id_str = ToString(lock_id);
+    debug_context.strarg1 = lock_id_str.c_str();
+  }
   RETURN_NOT_OK(FlushBufferedOperations(debug_context));
   tserver::PgAcquireAdvisoryLockRequestPB req;
   AdvisoryLockRequestInitCommon(req, pg_client_.SessionID(), lock_id, mode);
@@ -1379,9 +1383,13 @@ Status PgSession::AcquireObjectLock(const YbcObjectLockId& lock_id, YbcObjectLoc
     return Status::OK();
   }
   VLOG(1) << "Lock acquisition via shared memory not available";
+  std::string lock_id_str;
   YbcFlushDebugContext debug_context {};
   debug_context.reason = YbcFlushReason::YB_ACQUIRE_OBJECT_LOCK;
-  debug_context.strarg1 = yb_debug_log_docdb_requests ? ToString(lock_id).c_str() : nullptr;
+  if (yb_debug_log_docdb_requests) {
+    lock_id_str = ToString(lock_id);
+    debug_context.strarg1 = lock_id_str.c_str();
+  }
 
   return pg_txn_manager_->AcquireObjectLock(
       VERIFY_RESULT(FlushBufferedOperations(debug_context)), lock_id, mode);