[Tests] SummaryEndpoints — add tests for all 4 methods

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What to build

SummaryEndpointsTests.cs exists but is completely empty. Add unit tests for all 4 endpoint methods in SummaryEndpoints.cs:

• GetSummaries — GET /api/summary/
• SaveSummary — POST /api/summary/summary
• GetReadingNotes — GET /api/summary/{number}
• SaveReadingNotesMarkdown — POST /api/summary/{number}/markdown

Follow the existing pattern in NoteEndpointsTests.cs (xUnit + FluentAssertions + mocked IDataStorageService / ISummaryService).

Acceptance criteria

• Happy-path test for each of the 4 endpoint methods
• Not-found / empty response test for at least GetSummaries and GetReadingNotes
• Validation-error test for SaveSummary (e.g. missing required fields)
• All new tests pass in CI

Blocked by

None — can start immediately.

Priority

🔴 Critical — this is the most important test gap in the API layer