chore: remove unused pruneCollectionsFileStrict method

[detail-app]

Removes the plural-collections pruneCollectionsFileStrict public method
from IngestedFilePruneService along with the two tests that exclusively
exercised it and the imports/constant that were only used by those tests.

The method had no production callers. Git history shows it was introduced
with a caller in LocalDocsFileIngestionProcessor (commit e904faf), which
was then removed ~8 hours later by a refactor (commit 1263e64) that switched
the processor to the per-collection pruneCollectionFileStrict variant.
The orphaned method was left behind with no production references — only
tests. The superseding singular variant and all private helpers it shares
remain untouched.

• drop pruneCollectionsFileStrict and its Javadoc
• drop the two pruneCollectionsFileStrict-only tests and now-unused
  assertThrows, doThrow, inOrder, InOrder imports and
  BOOKS_COLLECTION_NAME constant

History

• Last materially changed July 2026 (feat(learning): ground guided lessons with durable citations #110 @WilliamAGH)

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