Expose Checker.getIndexInfoOfType on the unstable API

[Baptiste Jamin (baptistejamin)]

getIndexInfoOfType already exists on the TypeScript 6 checker and internally in the Go checker (Checker.GetIndexInfoOfType), but it is not wired through typescript/unstable. This PR exports it on the JS API.
Unlike the TS 6 surface (IndexKind), this matches the Go checker: it takes the index key type (checker.getStringType(), checker.getNumberType(), or another key type) and returns that index info, or undefined if none exists.
Needed by typescript-eslint (isTypeReadonly).

• API tests (async + sync): string index, missing index, readonly index

[Copilot]

Pull request overview

Exposes Checker.getIndexInfoOfType through the synchronous and asynchronous unstable JavaScript APIs.

Changes:

• Adds protocol routing and serialization for index-info lookups.
• Implements sync and async checker methods.
• Adds coverage for matching, missing, and readonly index signatures.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tsc/internal/api/session.go	Handles index-info requests.
tsc/internal/api/proto.go	Defines the protocol method and parameters.
packages/typescript/src/api/proto.generated.ts	Adds generated protocol typings.
packages/typescript/src/api/sync/api.ts	Exposes the synchronous API.
packages/typescript/src/api/async/api.ts	Exposes the asynchronous API.
packages/typescript/test/sync/api.test.ts	Tests synchronous behavior.
packages/typescript/test/async/api.test.ts	Tests asynchronous behavior.

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