Fix tests against axios 0.33

[anttiviljami]

Problem

The 0.*.* entry in the CI matrix now resolves to axios 0.33.0 (the 0.x line picked up 0.31–0.33 prototype-pollution hardening releases), and 30 of 69 tests fail there:

TypeError: Cannot read properties of undefined (reading 'name')
  at handleRequest (node_modules/axios-mock-adapter/src/handle_request.js:78:51)

Two distinct causes, one upstream change:

1. axios 0.33 builds utils.merge results with Object.create(null). dispatchRequest runs every request's headers through that merge, so config.headers reaches the adapter with a null prototype — and axios-mock-adapter reads config.headers.constructor.name === 'AxiosHeaders', which throws on the undefined constructor. The stack points at our runRequest line, but the fault is inside the mock adapter. axios-mock-adapter 2.1.0 has the identical check, so upgrading it does not fix this.
2. The same hardening makes mergeConfig hand back null-prototype defaults.auth and defaults.proxy. toStrictEqual compares prototypes, so it fails with "serializes to the same string" even though the values are unchanged.

The library itself behaves correctly under axios 0.33 — this is purely a test-tooling incompatibility.

Changes

• src/__tests__/setup.ts (new) — wraps MockAdapter.prototype.adapter and, only when config.headers has a null prototype, passes the handler a config with a normal-prototype copy of the headers. axios 1.x and 0.25/0.30 take the untouched path, so AxiosHeaders detection still behaves exactly as before.
• jest.config.ts — registers the setup file via setupFilesAfterEnv.
• src/client.test.ts — toStrictEqual → toEqual for the two prototype-sensitive defaults assertions.

Verification

npm test passes 69/69 on every matrix entry:

axios	result
0.25.0 (oldest supported)	69 passed
0.30.2	69 passed
0.33.0 (latest 0.x)	69 passed
1.18.1 (^1.0.0 / latest)	69 passed

npm run lint and npm run build are clean.

Note

The shim exists to work around an axios-mock-adapter bug present in both 1.22.0 and 2.1.0; worth filing upstream if we want to drop it eventually.

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