Add test coverage for GetMythologyById endpoint

[Copilot]

Review feedback on PR #1 identified missing test coverage for the GetMythologyByIdAsync endpoint. Added unit and integration tests following existing patterns.

Changes:

• Extracted inline lambda to named GetMythologyById method for testability
• Added 7 unit tests: success case, 404 handling, edge cases (zero/negative/max IDs, empty gods)
• Added 10 integration tests: full HTTP cycle, 404 responses, related entity loading, sequential requests

Pattern:

// Before: inline lambda (not unit testable)
mythologies.MapGet("{id}", async (int id, IMythologyRepository repository) =>
{
    var mythology = await repository.GetMythologyByIdAsync(id);
    return mythology is null ? Results.NotFound() : Results.Ok(mythology);
});

// After: named method (unit testable)
mythologies.MapGet("{id}", GetMythologyById);

public static async Task<IResult> GetMythologyById(int id, IMythologyRepository repository)
{
    var mythology = await repository.GetMythologyByIdAsync(id);
    return mythology is null ? Results.NotFound() : Results.Ok(mythology);
}

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