Fix RepeatedField::MergeFrom self-merge heap pointer disclosure (#27607)

## Summary

`RepeatedField<T>::MergeFrom` has an unsafe self-merge path. When `MergeFrom(self)` is called on a field at SOO capacity (2 elements for `int32_t`), `Reserve()` triggers a SOO→heap transition. The `other_is_soo` flag captured before `Reserve()` becomes stale — `other.elements(stale_true)` returns `soo_data_[]`, which now holds the raw `HeapRep*` pointer bytes. `UninitializedCopyN` then copies those bytes as `int32_t` values, appending two elements containing the low/high 32 bits of the heap address.

`ABSL_DCHECK_NE(&other, this)` caught this in debug builds but was compiled out with `-DNDEBUG`, so release builds proceeded silently and produced corrupted contents / heap-address disclosure.

## Fix

Self-merge is undefined behavior. Rather than silently no-op, this turns it into a well-defined termination: an out-of-line `internal::LogSelfMergeAndAbort()` that fires `ABSL_LOG(FATAL)` in all build modes.

```cpp
template <typename Element>
inline void RepeatedField<Element>::MergeFrom(const RepeatedField& other) {
  if (ABSL_PREDICT_FALSE(&other == this)) {
    PROTOBUF_NO_MERGE internal::LogSelfMergeAndAbort();
  }
  ...
```

The abort helper is declared out-of-line (in `repeated_field.cc`) so the failure path does not pull `ABSL_LOG` streaming support into every inlined `MergeFrom` instantiation. The previously-existing `ABSL_DCHECK_NE(&other, this)` is now dead code (the self-reference branch terminates before reaching it) and has been removed.

## Test

`RepeatedField.MergeFromSelfFailsWithATermination` exercises the SOO-capacity self-merge — the case that previously appended heap-pointer bytes in release builds — and asserts it now terminates:

```cpp
RepeatedField<int32_t> field;
field.Add(1);
field.Add(2);
EXPECT_DEATH(field.MergeFrom(field), "self-reference");
```

Closes #27607

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/27607 from adilburaksen:fix/repeated-field-self-merge cbe6a42d7ae2ed8474fd4226f1aca0d349e7c452
PiperOrigin-RevId: 927283856