`get self(): this` in an interface crashes the checker (6.0 regression)

Nobody has claimed this yet.

### 🔎 Search Terms

getConditionalFlowTypeOfType, getAnnotatedAccessorType, Cannot read properties of undefined (reading 'flags'), accessor this type

### 🕗 Version & Regression Information

- Crashes in 6.0.3 and nightly (6.0.0-dev.20260416); 5.9.3 is clean — a 6.0 regression

### 💻 Code

```ts
interface I {
get self(): this
}
```

### 🙁 Actual behavior

`tsc` crashes:

```
TypeError: Cannot read properties of undefined (reading 'flags')
at getConditionalFlowTypeOfType (typescript/lib/_tsc.js:65892:23)
at getTypeFromTypeNode (typescript/lib/_tsc.js:68622:12)
at getAnnotatedAccessorType (typescript/lib/_tsc.js:62162:20)
at getTypeOfAccessors (typescript/lib/_tsc.js:62180:99)
at checkAccessorDeclarationDiagnostics (typescript/lib/_tsc.js:87089:26)
```

Any `this` nested inside the accessor's return type annotation triggers it too (`get pair(): [this, string]`). A method or property signature with the same `this` type is fine.

### 🙂 Expected behavior

No crash (5.9 checks this without errors).

### Additional information about the issue

_No response_

Reproduce the crash with the interface snippet and the reported accessor cases. Start at getAnnotatedAccessorType, getTypeOfAccessors, and getConditionalFlowTypeOfType in the reported checker stack, then locate the corresponding TypeScript sources and regression-test setup. Done means these accessors no longer crash and the existing 5.9 behavior remains covered by a test.

Written by the indexing model from the issue text.