feat: expose fetch option on ManagementClient

[harshithRai]

Changes

Exposes a fetch option on ManagementClient so consumers can supply their own HTTP transport. The internal fetcher layer (#1238) already routes through fetcherImpl(args.fetchFn ?? …), but the public wrapper both omitted fetch from ManagementClientOptions and explicitly deleted it in the constructor; so there was no supported way to plug one in.

• ManagementClientOptions: drop "fetch" from the Omit<FernClient.Options, …> list so the field is part of the public surface.
• ManagementClient constructor: stop deleting _options.fetch. The internal fetcher is still stripped (Fern implementation detail), with a comment explaining why and linking Adding support for fetch options in V5? #1330.
• Add an enriched JSDoc on the fetch field (proxies/egress, retry middleware, OpenTelemetry, Cloudflare Workers/Bun).

Source-compatible: existing call sites that don't pass fetch are unaffected. Callers can now opt in:

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

References

Please include relevant links supporting this change such as a:

• Closes Adding support for fetch options in V5? #1330

Testing

New unit test in src/management/tests/unit/management-client-fetch.test.ts:

1. Constructs a ManagementClient with a custom fetch, calls client.rules.list(), and asserts the custom fetch was invoked against tenant.auth0.com/api/v2/rules and that the parsed return value matches the payload it served.
2. Constructs a ManagementClient without fetch to confirm the default global-fetch route is unchanged.
   Verified the test fails to compile without the source change (regression guard). tsc --noEmit clean; existing unit tests pass.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.75%. Comparing base (a823058) to head (193c32a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1372      +/-   ##
==========================================
- Coverage   89.76%   89.75%   -0.01%     
==========================================
  Files         431      431              
  Lines       20108    20107       -1     
  Branches     9782     9452     -330     
==========================================
- Hits        18049    18048       -1     
  Misses       2059     2059              

Flag	Coverage Δ
alltests	89.75% <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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[harshithRai]

closing as duplicate of #1352