feat: Add Tags to Photo entity (opt-in via PhotoSearchExtras.Tags)

[toralux]

Summary

Add a nullable Tags property to the Photo entity, mapped to the Flickr API tags field.

Background

Flickr's flickr.photos.search response includes a space-delimited tags field when the caller's extras includes PhotoSearchExtras.Tags. The library already has the PhotoSearchExtras.Tags flag, but the Photo record never maps the returned tags field — so it's silently dropped during deserialization.

This follows the exact same pattern and precedent as the previously merged Description addition (commit 5f2a23aac1ed112aec1b0aa0df7e5b980e37dd0e): a new nullable property mapped to a Flickr API field that is only present in the response when its corresponding PhotoSearchExtras flag is requested.

Changes

• src/Flickr.Net/Entities/Photo.cs: Added Tags property (string?, [JsonPropertyName("tags")])
• src/Flickr.Net.Test/Entities/PhotoTests.cs: Added PhotoTagsAreDeserialized and PhotoTagsIsNullWhenNotPresent tests, following the existing PhotoDescription* test pattern

Compatibility

Fully backward compatible:

• The property is nullable and defaults to null
• No payload or behavior change for callers who don't include PhotoSearchExtras.Tags in their extras
• No breaking changes to existing API surface

Test results

All 88 tests pass (86 existing + 2 new).

[st0o0]

LGTM!

[toralux]

@LineOfC0d3 Any chance to have this merged and published as a new nuget-version? Thanks :-)