Incorrect emitted declaration for unique symbol re-assignment

### 🔎 Search Terms

"TS1332", "unique symbol readonly", "function readonly property"

### 🕗 Version & Regression Information

N/A.

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20250819#code/MYewdgzgLgBAtgTwMoLgIxAGxgXhi9LACgHICNMIA6RcrEgSgG4AoF0SWOy3GAbxYwh8ZKgosAvjACGEGB2isWAMwCuYYFACW4EQDF1mnWCIN+EtogMbt4GqMLY83arTFYlAegBULJAFF-GAADKgARKgAVJGCYSIBBACEqOIAJIKSAeQA1IIBhTLCggAUAJUKAVTz-JAAuNgATAFNgTGkAJyaYMGk4JogAB2lgLqtDWzB+QWEANw6RblqYdS0AR1UuiHdMVgsWAHVUgEk81JgjpBh4gDkYf1Ly0pgiaIBGAGZ3gCYl+Jg59paaRoTBdADuAAsQBAulAEAMulo5NIYCQVutNtsSDA4Qj4KpoDA0F0FFAGCxvJ42E0AB4DEDtWB8MY2YwWIA

### 💻 Code

```ts
const mySymbol = Symbol('Symbols.mySymbol');

const Symbols = {
mySymbol
} as const;

function myFunction() {}

myFunction.mySymbol = Symbols.mySymbol;

/*
SEE `.D.TS` TAB. THE ABOVE CODE PRODUCES:

declare namespace myFunction {
var mySymbol: unique symbol;
}

WHICH IS AN ERROR (TS1332: A variable whose type is a 'unique symbol' type must be const)
*/

export {myFunction}
```

### 🙁 Actual behavior

Compiler produces:

```ts
declare namespace myFunction {
var mySymbol: unique symbol;
}
```

### 🙂 Expected behavior

Should be:

```ts
declare namespace myFunction {
const mySymbol: unique symbol;
}
```

### Additional information about the issue

Behaves differently when not in a module (remove `export {myFunction}` in the Playground)

Start with the linked Playground reproduction and inspect declaration emission for the assigned property on an exported function, comparing module and non-module behavior. The fix is done when the emitted declaration uses `const mySymbol: unique symbol` rather than `var`, while preserving the reported output in the other case.

Written by the indexing model from the issue text.