Use concreteType for SafeDIMockConfiguration struct extensions

[dfed]

Summary

• Protocol extensions cannot contain nested type declarations
• When a dependency's instantiated type is a protocol (e.g., AuthService fulfilled by DefaultAuthService), the generated SafeDIMockConfiguration must be nested in the concrete type's extension, not the protocol's
• Reverts Revert leaf mock wrapper generation #238's change from concreteType to instantiatedTypeDescription for configurationTypeName and dedup key
• This bug exists on main but is rarely triggered — protocol-typed deps that needsConfigurationStruct (have children) are uncommon without the onlyIfAvailable tree-threading changes

Test plan

• All 742 tests pass
• SwiftFormat lint passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (1f0bf01) to head (cceb140).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #243      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files           41       41              
  Lines         5772     5772              
===========================================
- Hits          5772     5771       -1     
- Misses           0        1       +1     

Files with missing lines	Coverage Δ
Sources/SafeDICore/Generators/ScopeGenerator.swift	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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