[Compatibility]: MAKEDATE returns VARCHAR instead of DATE

## Environment

- MatrixOne: latest `main`, commit `f0c31cd4b830be32442cf329e0a3fb08aa9c16c3`
- MySQL comparison: 8.3.0

## Problem

`MAKEDATE(year, dayofyear)` returns an unbounded `VARCHAR` in MatrixOne instead of a `DATE`. The displayed text looks like a date, but the SQL result domain, wire-protocol metadata, derived schema, and downstream type propagation are all wrong.

## Reproduction

```sql
SELECT MAKEDATE(2024, 60) AS d;

CREATE TABLE copied AS
SELECT MAKEDATE(2024, 60) AS d;

SHOW CREATE TABLE copied;
```

MatrixOne reports the scalar as MySQL protocol type 253 (`VAR_STRING`) with length 65535 and creates:

```sql
CREATE TABLE `copied` (
`d` varchar(65535) DEFAULT NULL
)
```

MySQL 8.3 reports protocol type 10 (`DATE`) and creates:

```sql
CREATE TABLE `copied` (
`d` date DEFAULT NULL
)
```

## Downstream effects

The incorrect string domain propagates through other date-capable expressions:

```sql
SELECT MAKEDATE(2024,60) + INTERVAL 1 DAY;
SELECT GREATEST(MAKEDATE(2024,60), CAST('2024-03-01' AS DATE));
SELECT MAKEDATE(2024,60) AS d UNION ALL SELECT CAST('2024-03-01' AS DATE);
```

MatrixOne reports string metadata for each result; MySQL keeps a date result. A view over `MAKEDATE` likewise exposes `VARCHAR(65535)` in `information_schema.columns` instead of `DATE`.

## Coverage

The following paths were repeated three times:

- valid and invalid scalar calls;
- text-protocol result metadata;
- SQL prepared statement execution and metadata;
- date arithmetic, `YEAR`, comparison, `GREATEST`, and `UNION`;
- view metadata and CTAS schema/value persistence.

An explicit `CAST(... AS DATE)` control is reported as protocol type 10 by both systems. The server remains healthy after all runs.

## Code analysis

The `MAKEDATE` registration in `pkg/sql/plan/function/list_builtIn.go` declares:

```go
args: []types.T{types.T_varchar, types.T_varchar},
retType: func(parameters []types.Type) types.Type {
return types.T_varchar.ToType()
},
```

`MakeDateString` in `pkg/sql/plan/function/func_binary.go` formats the date and appends bytes to a varlen result. There is no typed `DATE` result path, so every downstream consumer sees a string rather than a date.

## Duplicate check

#26830 included `MAKEDATE` only as one example of incorrect nullability for NULL-producing functions; it did not cover the function's return domain and is closed. This issue is independent.