Fix the upstream format support + add tests & examples

[rdimitrov]

The following PR:

• Fixes the upstream registry support (it was using a legacy format), now it matches what toolhive-registry generates
• Adds example registry files with a 10-12 servers inside it for testing/development
• Removes the legacy registry example

[codecov-commenter]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.09%. Comparing base (7f9d821) to head (a668cd1).

Files with missing lines	Patch %	Lines
internal/sources/types.go	54.54%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #201 +/- ## ========================================== - Coverage 68.24% 68.09% -0.16%  ========================================== Files 59 59 Lines 3940 3940 ========================================== - Hits 2689 2683 -6  - Misses 1079 1082 +3  - Partials 172 175 +3 

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