[client-v2] readVariant discards the active type discriminant — colliding Variant alternatives are indistinguishable

## Description

`BinaryStreamReader.readVariant(ClickHouseColumn)` reads the 1-byte Variant discriminant (`ordNum`) only to select which nested column's reader to run, then returns **just the decoded value** and throws the discriminant away:

```java
// client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java:892
public Object readVariant(ClickHouseColumn column) throws IOException {
int ordNum = readByte() & 0xFF;
if (ordNum == 0xFF) {
return null;
}
return readValue(column.getNestedColumns().get(ordNum)); // ordNum (the active type) is discarded
}
```

For many Variants the active alternative can be inferred from the returned object's runtime class, but for alternatives whose readers produce the **same Java runtime type** the information is lost, and the consumer cannot recover which ClickHouse subtype was actually on the wire. Examples where the decoded objects are indistinguishable:

| Variant | both alternatives decode to | why ambiguous |
| --- | --- | --- |
| `Variant(DateTime, DateTime64(3))` | `java.time.ZonedDateTime` (lines 186-191 → `convertDateTime`, `typeHint == null` → returns `ZonedDateTime`) | same class, same instant |
| `Variant(String, FixedString(N))` | `java.lang.String` | same class |
| `Variant(Decimal32(s), Decimal64(s))` | `java.math.BigDecimal` | same class/scale |

Because `readVariant` returns a bare `Object`, a downstream consumer (text/JSON renderer, type-aware formatter, or any caller inspecting the value) has no way to choose the correct subtype for formatting in these cases.

This is the Java analogue of ClickHouse/clickhouse-js#910. Note the *specific* examples in that JS issue do **not** collide here — `clickhouse-java` maps `Enum8`/`Enum16` to a dedicated `EnumValue` (lines 172-181) and `Date` to `LocalDate` (line 183), so `Variant(UInt8, Enum8(...))` and `Variant(Date, DateTime)` are recoverable from the runtime type. The underlying limitation — the discriminant index is not surfaced — is the same, and other type pairs (above) hit it.

## ClickHouse server version

26.6.1.1193 (collision behavior determined by code analysis of the type→class mapping; not exercised end-to-end against the server).

## Reproduction

A `Variant(DateTime, DateTime64(3))` column: whichever alternative the server picks, `readVariant` returns a `ZonedDateTime`, so the caller cannot tell `DateTime` from `DateTime64`.

```java
// Conceptual unit-level repro against BinaryStreamReader
ClickHouseColumn col = ClickHouseColumn.of("v", "Variant(DateTime, DateTime64(3))");

// Wire bytes: discriminant 0x01 selects the DateTime64 alternative (alternatives are
// sorted by ClickHouse's global type-name ordering), followed by its encoded value.
byte[] payload = /* 0x01 ++ encoded DateTime64 value */;
BinaryStreamReader reader = new BinaryStreamReader(
new ByteArrayInputStream(payload), null, LZ4_FACTORY, null);

Object value = reader.readVariant(col);

// EXPECTED: caller can determine the active alternative was DateTime64 (index 1)
// ACTUAL: value is a java.time.ZonedDateTime, identical to what the DateTime
// alternative (index 0) would have produced — the active type is unrecoverable.
assertTrue(value instanceof ZonedDateTime); // passes for BOTH alternatives
```

The same happens through the high-level reader path (`readValue` → `case Variant` at line 248-250): reading a `Variant(String, FixedString(10))` column yields a `String` with no indication of which alternative it was.

## Suggested fix

Surface the active alternative index alongside the value rather than discarding it — mirroring the upstream proposal. Options:
- Return a small wrapper, e.g. `VariantValue { int typeIndex; Object value; }`, from `readVariant` (breaking change to the public `Object readVariant(...)` signature — gate behind a minor/major bump), or
- Expose the resolved subtype (`column.getNestedColumns().get(ordNum)`) so a renderer can format with the exact `ClickHouseColumn`.

Buggy code: `BinaryStreamReader.readVariant` at `client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java:892-898` (dispatched from `readValue` `case Variant` at line 248-250). The legacy `clickhouse-data` RowBinary processor should be checked for the same pattern.

## Link

Relayed from ClickHouse/clickhouse-js#910.

Bắt đầu trong client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/BinaryStreamReader.java tại readVariant, khoảng các dòng 892-898, sau đó lần theo trường hợp Variant trong readValue, khoảng các dòng 248-250. So sánh các cách tiếp cận wrapper hoặc resolved-subtype được đề xuất và kiểm tra bộ xử lý RowBinary legacy của clickhouse-data để tìm cùng một mẫu. Hoàn thành nghĩa là các caller có thể phân biệt những lựa chọn xung đột như DateTime và DateTime64, với các bài kiểm thử bao phủ discriminant.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.