fix: make MockAgent work with protocol-aware global dispatchers

[mcollina]

Summary

Fixes the regression where setGlobalDispatcher(new MockAgent()) no longer intercepts Node.js native fetch() requests.

The original regression happened after legacy global-dispatcher consumers started forcing allowH2: false. That made Agent route those requests through a separate HTTP/1-only path, while MockAgent only registered mocks for the default origin path.

This PR does more than mirror a second mock bucket:

• makes Agent explicitly protocol-aware internally by separating
  • default dispatcher entries
  • HTTP/1-only dispatcher entries
• adds internal helper symbols for querying and mutating protocol-specific dispatcher entries
• updates MockAgent to use those helpers instead of sharing or depending on Agent's raw pool map
• keeps mock interceptor state attached to the logical mock registry while registering protocol-specific dispatchers into Agent
• adds regression coverage for native fetch() and protocol-specific mock registration

This removes the brittle coupling between Agent and MockAgent around the HTTP/1-only path and preserves native fetch() interception.

Fixes: #5036

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.87006% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.08%. Comparing base (d64e7bb) to head (72c3bad).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/mock/mock-agent.js	97.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5052      +/-   ##
==========================================
+ Coverage   93.03%   93.08%   +0.04%     
==========================================
  Files         110      110              
  Lines       35786    35934     +148     
==========================================
+ Hits        33294    33449     +155     
+ Misses       2492     2485       -7     

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[metcoder95]

just conflicts on files