feat(period-score): expose the games won inside a period

[dsaiko]

Why

Set-based classic sports report the games of each set on the period row, next to
the running sets-won tally. A live tennis match with the first set won 6:4 and
the second under way arrives from the feed as:

<sport_event_status status="1" match_status="201" scoreboard_available="true"
                    home_score="1" away_score="0">
  <period_score type="set" number="1" match_status_code="200"
                home_score="1" away_score="0" home_games="6" away_games="4"/>
  <period_score type="set" number="2" match_status_code="201"
                home_score="1" away_score="0" home_games="0" away_games="1"/>
  <scoreboard home_points="40" away_points="40" home_games="0" away_games="1"/>
</sport_event_status>

home_games / away_games had no place in the period score models — games
existed only on the scoreboard, which describes the set in progress and nothing
else. XmlSerializer therefore dropped the attribute while decoding, and a
consumer was left with home_score: the sets-won tally, identical on every row
(1:0, 1:0, …). Rendering a per-set row from it shows 1:0 where the set was 6:4.

It also cannot be reconstructed downstream. The scoreboard resets its games with
each new set, so once a set ends its score is gone from the live state, and a
consumer restarting mid-match has nothing to replay it from.

What changed

• home_games / away_games on periodScoreType (feed) and periodScore
  (API), using the Specified pattern already used elsewhere in these models —
  XmlSerializer cannot encode nullable attributes (the reverted elapsed_time
  nullable fix hit exactly this), and the flag keeps absence distinguishable
  from a legitimate 0:0 at the start of a set
• HomeGames / AwayGames as int? on IPeriodScore and the PeriodScore
  entity
• mapped in both cache paths, feed and API (Specified ? value : null)
• new Oddin.OddsFeedSdk.Tests project (xunit) — the repo had none; the CI
  "Run Library Tests" step was a placeholder pointing at the library project
  and now runs the test project instead

Additive only — no existing field changes shape or meaning.

Mirrors the same change in gosdk: oddin-gg/gosdk#48

Tests

PeriodScoreGamesTests deserializes the payload above through both decode
paths (odds_change and match_summary) and asserts a completed set keeps
6:4, the set in progress carries 0:1, a period without games leaves the
Specified flags unset (→ null on the entity), and the sets-won tally on the
same row stays readable and independent.

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