Drop tautological test_sources.py

The tests asserted invariants the data structure already guarantees: dict
keys match source.name because the factory literally builds it that way,
pinned_sources() contains only pinned entries because that's its filter,
dataclass fields are non-empty because they're defaulted, etc. Per the
repo's testing guidance, tests must fail on wrong behavior, not on
implementation changes — these failed on either.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ravwojdyla