fix: use assertion instead of modifying schema under shared lock (#46242)

Related to #46225

Replace the heterogeneous insert data handling logic that modified
schema_ while holding a shared lock with an assertion. The previous
implementation had a concurrency bug where schema modification
operations were performed under a shared_lock, which violates mutex
semantics and can lead to data races.

Issue: #46225 reported two problems:
1. Schema modification under shared_lock (not exclusive lock)
2. Access to schema_ not protected by mutex in growing segment

The removed code attempted to handle "added fields" by:
- Adding new field to schema (schema_->AddField)
- Appending field metadata to insert_record_
- Setting default data for existing rows

All these write operations were performed while holding only a
shared_lock, which is incorrect since shared_locks are meant for
read-only operations.

This fix replaces the unsafe modification with an assertion that fails
if an unexpected new field is encountered in a growing segment with
existing data. The proper handling of schema changes should go through
the Reopen() path which correctly acquires a unique_lock before
modifying schema_.

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/core/src/segcore/SegmentGrowingImpl.cpp

@@ -108,26 +108,10 @@ SegmentGrowingImpl::Insert(int64_t reserved_offset,
         auto field_id = FieldId(field.field_id());
         AssertInfo(!field_id_to_offset.count(field_id), "duplicate field data");
         field_id_to_offset.emplace(field_id, field_offset++);
-        // may be added field, add the null if has existed data
-        if (exist_rows > 0 && !insert_record_.is_data_exist(field_id)) {
-            LOG_WARN(
-                "heterogeneous insert data found for segment {}, field id {}, "
-                "data type {}",
-                id_,
-                field_id.get(),
-                field.type());
-            schema_->AddField(FieldName(field.field_name()),
-                              field_id,
-                              DataType(field.type()),
-                              true,
-                              std::nullopt);
-            auto field_meta = schema_->get_fields().at(field_id);
-            insert_record_.append_field_meta(
-                field_id, field_meta, size_per_chunk(), mmap_descriptor_);
-            auto data = bulk_subscript_not_exist_field(field_meta, exist_rows);
-            insert_record_.get_data_base(field_id)->set_data_raw(
-                0, exist_rows, data.get(), field_meta);
-        }
+        AssertInfo(exist_rows == 0 || insert_record_.is_data_exist(field_id),
+                   "unexpected new field in growing segment {}, field id {}",
+                   id_,
+                   field.field_id());
     }
 
     // segment have latest schema while insert used old one