[gaxios] `undici-types` is not declared as dependency in package.json

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### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://github.com/chrispisano-bc/bare-bones-bazel

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

### Environment details

- OS: macOS 15.7.4 (24G517)
- Bazel with aspect_rules_ts 2.4.2
- aspect_rules_ts 3.6.3 (TypeScript project)
- Node/npm: 22.11.0
- pnpm 9.14.2

### Steps to reproduce

1. Clone this repo: https://github.com/chrispisano-bc/bare-bones-bazel
2. read `REPRODUCE.MD`
3. Observe that the type checker cannot resolve undici-types: `error TS2307: Cannot find module 'undici-types' or its corresponding type declarations.`
4. Observe that if you install `undici-types` as a direct dependency or flip `skipLibCheck` to `true`, then the `reproduce.js` script no longer throws.

### A clear and concise description of what the bug is, and what you expected to happen.

### Description

undici-types is not declared as a direct/optional dependency in gaxios’s package.json, but the published type declarations use it. In [packages/gaxios/src/common.ts](https://github.com/googleapis/google-cloud-node-core/blob/7feb4c8cc253c87f8da6ecb078b096a896b4d840/packages/gaxios/src/common.ts#L30-L32) (and in the emitted build/cjs/src/common.d.ts), the type is defined as:
```
type _BodyInit = typeof globalThis extends {BodyInit: infer T}
? T
: import('undici-types').BodyInit;
```

In an isolated build environment (Bazel), this breaks the build if the associated tsconfig file has set `skipLibCheck: false`

### Expected Result
Type-checking succeeds in isolated build environments. Gaxios should either declare `undici-types` as an optional/direct dependency or the published types should not reference it.

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

I expect library types to compile even when `skipLibCheck` is set to `false`