feat: expose fetch option on ManagementClient

[tsushanth]

Why

Closes #1330.

The internal fetcher layer added `BaseClientOptions.fetch?: typeof fetch` in #1238 and routes it through `fetcherImpl(args.fetchFn ?? …)`. The public wrapper, however, both omitted `fetch` from `ManagementClientOptions` and explicitly deleted it from the incoming options bag, so library consumers had no supported way to provide their own HTTP transport — even though the SDK is already capable of using one. Typical use cases the reporter and downstream Auth0 users have called out: proxies / corporate egress, retry middlewares, OpenTelemetry instrumentation, and Cloudflare Workers / Bun targets where the global `fetch` needs swapping.

What

• Drop `"fetch"` from the `Omit<FernClient.Options, …>` list so the field is part of the public `ManagementClientOptions` surface.
• Stop deleting `_options.fetch` in the `ManagementClient` constructor. The internal `fetcher` is still stripped (it's a Fern implementation detail), with a comment explaining why and linking back to this issue.

Tests

`tests/management/ManagementClient.test.ts` — two cases:

1. Constructs a `ManagementClient` with a custom `fetch` implementation, fires `client.rules.list()`, asserts the custom function was invoked at least once and that the URL targets `tenant.auth0.com/api/v2/rules`. Without the wrapper change this fails because `fetch` is silently dropped before reaching `fetcherImpl`.
2. Constructs a `ManagementClient` without `fetch` to confirm the default global-fetch route is unchanged.

`npx jest tests/management/ManagementClient.test.ts` — both pass. `npx tsc --noEmit` clean. `eslint` clean.

Compat

Source-compatible: existing call sites that didn't pass `fetch` see no behavioural difference. Callers can now opt in:

```ts
new ManagementClient({
domain: 'tenant.auth0.com',
token: '...',
fetch: myInstrumentedFetch,
});
```

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (71a98da) to head (5e96b0b).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1352      +/-   ##
==========================================
- Coverage   89.62%   89.62%   -0.01%     
==========================================
  Files         426      426              
  Lines       19804    19803       -1     
  Branches     9716     9359     -357     
==========================================
- Hits        17749    17748       -1     
  Misses       2055     2055              

Flag	Coverage Δ
alltests	89.62% <ø> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/management/wrapper/ManagementClient.ts	100.00% <ø> (ø)

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