fix(mock): native fetch bypasses MockAgent due to allowH2

[Megashubham]

Fixes #5036

Description

This PR fixes an issue where native Node.js fetch queries would bypass the MockAgent globally registered using setGlobalDispatcher.

Root Cause

In version 8.0.3, a change was introduced to wrap the legacy global dispatcher with a Dispatcher1Wrapper (fix: mirror the legacy global dispatcher for built-in fetch). To support legacy consumers, Dispatcher1Wrapper.dispatch modifies opts to forcefully add allowH2: false.

Because of this, native node fetch passes allowH2: false when delegating to the MockAgent. When MockAgent proxies the request to its internal encapsulated Agent instance:

1. MockAgent.get(origin) pre-registers and creates a MockClient for the origin exactly as-is.
2. The encapsulated Agent.dispatch checks allowH2: false and modifies the lookup key to be ${origin}#http1-only.
3. Because the MockClient registered is under ${origin}, the cache miss causes the underlying Agent to instantiate a brand new real Client under ${origin}#http1-only.
4. As a result, the request completely bypasses the mock interceptors and is fired over the real network!

Fix

The mock interceptor system shouldn't be concerned about whether HTTP/2 or HTTP/1.1 is being explicitly requested by the wrapper. To solve this, MockAgent.dispatch now safely deletes the allowH2 flag from dispatchOpts if it is set to false. This ensures the underlying encapsulated Agent retains the standard origin as the lookup key and successfully resolves to the pre-registered MockClient.

A regression test using native globalThis.fetch has also been added in test/issue-5036.js.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (0f1f890) to head (e9e49a0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5448      +/-   ##
==========================================
- Coverage   93.46%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       37106    37110       +4     
==========================================
+ Hits        34682    34685       +3     
- Misses       2424     2425       +1     

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[Megashubham]

Can you check once :P
@metcoder95