[YevheniiKotyrlo]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: isPropertyReadonlyInType misreads a property an interface inherits from a mapped type #1056
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

An interface inherits a mapped type's property symbol as-is, so isPropertyReadonlyInType asked the interface — which isn't the mapped type — and fell back to the declaration the mapped type mapped.
For a class or interface, isReadonlyPropertyFromBaseType now finds the base type that supplies the same symbol (typeChecker.getBaseTypes) and asks about that base type instead, after the mapped-type check.

The new tests cover Readonly<Box>, Readonly<Pick<Box, "value">>, a chain of two interfaces, the readonly base among several base types, a -readonly mapping of a readonly declaration, and Partial<Box>; the first five fail without the change.
All CI steps pass locally on Node 26, with the suite also run on TypeScript 5.5.4 and 5.9.3.

#1056 isn't triaged yet, so the second and third boxes stay unchecked; I'm sending the fix now so it can be read alongside the report.

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