typespec-ts: stop relying on api-extractor internal `_defaultConfig` in spector test harness

## Problem

`packages/typespec-ts/test/commands/run.ts` builds an api-extractor config in memory (no `api-extractor.json` on disk) and relies on the private `ExtractorConfig._defaultConfig` static to get the built-in defaults:

```ts
structuredClone((ExtractorConfig as any)._defaultConfig)
```

In `@microsoft/api-extractor` 7.59.0, this internal was moved from a static class property to a module-level constant, so `._defaultConfig` returns `undefined` and the config merge crashes with:

```
TypeError: Cannot read properties of undefined (reading 'apiReport')
```

This breaks every declaration-rollup emit in the Spector e2e job.

## Current workaround

PR #5212 pins `@microsoft/api-extractor` to `~7.58.1` via an override in `pnpm-workspace.yaml` to avoid resolving 7.59.0.

## Proposed fix

Replace the `_defaultConfig` access with one of:
- Loading `schemas/api-extractor-defaults.json` directly via `JsonFile.load` from `@rushstack/node-core-library` (the file is JSONC, so plain `JSON.parse` won't work)
- Using a supported public API if one exists for getting default config values
- Restructuring the test to use `ExtractorConfig.loadFile` with a temporary config file

Once fixed, the `@microsoft/api-extractor` override can be removed from `pnpm-workspace.yaml`.

Start in packages/typespec-ts/test/commands/run.ts and inspect how the in-memory api-extractor configuration obtains defaults; review schemas/api-extractor-defaults.json and the pnpm-workspace.yaml override. Run the Spector declaration-rollup emit with api-extractor 7.59.0. Done means the emit succeeds without accessing _defaultConfig and the override can be removed.

Written by the indexing model from the issue text.