fix: add typesVersions so testing module resolves under classic TypeScript moduleResolution

[64johnlee]

Problem

The ./testing subpath export is declared in exports correctly, but TypeScript's default moduleResolution: "node" does not read the exports field in package.json. This means projects using the default (or older) TypeScript configuration get a type error when importing the testing utilities:

import { getTestServer } from '@google-cloud/functions-framework/testing';
// Error: Module '@google-cloud/functions-framework/testing' not found

Both getTestServer and getFunction are affected.

Root Cause

TypeScript only respects exports subpath maps when moduleResolution is set to "node16", "nodenext", or "bundler". Most projects still use "node" (the classic resolver), which only looks at main and types at the package root.

Fix

Add a typesVersions field, which is the standard way to declare subpath type mappings for all TypeScript versions and moduleResolution modes:

"typesVersions": {
  "*": {
    "testing": ["./build/src/testing.d.ts"]
  }
}

This is additive — projects already using node16/bundler continue using the exports map; projects using classic node resolution now also work.

Verification

After this change, the following import works without moduleResolution: "node16":

import { getTestServer, getFunction } from '@google-cloud/functions-framework/testing';

Fixes #594

🤖 Generated with Claude Code

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