Add HasDataChangedAsync for lightweight change detection via HEAD request

[jfversluis]

Summary

Adds a new HasDataChangedAsync method to ISessionizeApiClient that uses Sessionize's built-in ?hashonly=true query parameter to fetch a lightweight hash of the data for any view endpoint, avoiding a full data download.

Motivation

Mobile and bandwidth-constrained apps (e.g., conference companion apps built with .NET MAUI) need to periodically check for schedule updates. The current approach requires a full GET request (~300KB for the All view). Sessionize supports a ?hashonly=true query parameter that returns only a ~40-byte SHA-1 hash of the data, which changes whenever the underlying data changes.

This is especially valuable for:

• Mobile apps checking for updates on resume/startup
• Apps implementing cache-first patterns with background refresh
• Reducing unnecessary API traffic for unchanged data

How it works

Appending ?hashonly=true to any Sessionize view endpoint returns just the hash:

GET https://sessionize.com/api/v2/{apiId}/view/All?hashonly=true
→ aa94b4bbb12bdbcbb9b5059e7dfa37818ebd5f8e  (40 bytes, ~50ms)

GET https://sessionize.com/api/v2/{apiId}/view/All
→ Full JSON payload (~300KB)

Each view (All, Sessions, Speakers, GridSmart, SpeakerWall) returns its own hash.

Changes

New DTO

• DataChangedResponse(bool HasChanged, string Hash) — a record containing whether data changed and the current server hash

Interface addition

• ISessionizeApiClient.HasDataChangedAsync(viewName, lastKnownHash?, sessionizeApiId?, cancellationToken?)
  • viewName: The Sessionize view to check (e.g., "All", "Sessions", "Speakers", "GridSmart", "SpeakerWall")
  • lastKnownHash: The hash from a previous call. If null, returns HasChanged=true with the current server hash
  • Same sessionizeApiId and cancellationToken pattern as existing methods

Implementation

• Sends a GET request to {apiId}/view/{viewName}?hashonly=true
• Reads the plain-text hash response
• Compares against the provided lastKnownHash (case-insensitive)
• Reuses the existing GetViewEndpoint() helper for consistent URL construction
• Follows existing logging patterns with ILogger

Tests

• 7 unit tests — mocked HttpClient verifying request URL construction, hash comparison scenarios (no previous hash, different hash, matching hash, case-insensitive comparison, empty response, explicit API ID)
• 6 integration tests — live API calls verifying real hash behavior including round-trip consistency and different hashes per view

Usage Example

// First call — no previous hash, gets the current hash
var result = await client.HasDataChangedAsync("All");
// result.HasChanged == true, result.Hash is the server's current hash

// Store result.Hash in your cache...

// Subsequent call — compare against stored hash
var check = await client.HasDataChangedAsync("All", storedHash);
if (check.HasChanged)
{
    // Download full data only when needed
    var data = await client.GetAllDataAsync();
    storedHash = check.Hash; // update stored hash
}

Backward Compatibility

This is a purely additive change:

• New method added to ISessionizeApiClient (interface addition)
• New DataChangedResponse DTO
• No changes to existing methods or behavior