Expose underage on IPlayer

[yung-madamm]

What & why

The REST player profile carries underage since oddsfeedschema v1.1.0, with the same -1 unknown / 0 no / 1 yes encoding as competitors. This reads it and exposes it on IPlayer.

• player_profilePlayer gets an underage XML attribute (string, as xsd.exe renders the competitor one).
• New public UnderageStatus enum (Unknown = -1, No = 0, Yes = 1) in Oddin.OddsFeedSdk.API.Entities.Abstractions, and an internal UnderageStatusParser; anything unexpected, including null, maps to Unknown.
• IPlayer.Underage / Player.Underage, backed by a single value on LocalizedPlayer (it is locale-independent). Unknown when the player cannot be loaded, matching how the other properties degrade.
• Players nested in a competitor profile share player_profilePlayer and PlayerCache.RefreshOrInsertItem, so they carry it too.

ICompetitor.Underage stays a string; changing it would be a breaking API change.

Verification

PlayerUnderageTests: parser theory (1, 0, -1, null, empty, junk), LocalizedPlayer default, and XmlHelper.TryDeserialize of a profile with underage="1". dotnet build (Release) 0 errors and dotnet test 8/8 on the .NET 6 SDK image.

🤖 Generated with Claude Code

Review follow-up (second commit)

• Omitted attribute no longer erases a known value. Confirmed against the code: HandlePlayersData feeds nested player_profilePlayer objects from every competitor profile response straight into RefreshOrInsertItem, and the culture is then marked loaded, so an unconditional assignment let a second-culture or nested payload without the attribute reset a known Yes/No to Unknown until the TTL. RefreshOrInsertItem now assigns only when data.underage is non-null; an explicit -1 still retracts.
• Cache path is tested end to end. CacheCarriesUnderageThroughToThePlayerAndKeepsItWhenAPayloadOmitsIt side-loads a competitor profile through the existing PublishOnlyApiClientProxy harness, reads Player.Underage (Yes), publishes a second culture without the attribute (still Yes), then an explicit -1 (Unknown).

Verified again on the .NET 6 SDK image: build 0 errors, PlayerUnderageTests 9/9.