types: expose PendingInterceptor and PendingInterceptorsFormatter on the MockAgent namespace

[RaphaelFakhri]

Fixes #5563.

Both interfaces are declared at the top level of types/mock-agent.d.ts and never exported, so a consumer writing a helper around pendingInterceptors(), or a custom pendingInterceptorsFormatter, has no way to name the argument type and ends up copying the declaration into their own code, where it then drifts from this one.

They move into the MockAgent namespace, next to Options and the other public members, and the two class members now point at the namespaced names. Nothing else in types/ or index.d.ts referenced them, so this is additive for consumers and a no-op for the existing declarations.

test/types/mock-agent.test-d.ts gains an assertion that both names are reachable as MockAgent.PendingInterceptor and MockAgent.PendingInterceptorsFormatter from outside the module.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (fc3450d) to head (ae24422).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5721      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.02%     
==========================================
  Files         110      110              
  Lines       38908    38908              
==========================================
- Hits        36368    36364       -4     
- Misses       2540     2544       +4     

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