fix(api): decode underage on player profiles

[yung-madamm]

What & why

oddsfeedschema declares underage on the player type from v1.1.0 (oddin-gg/oddsfeedschema#22), with the same -1 unknown / 0 no / 1 yes encoding as competitor@underage. internal/schemacheck follows oddsfeedschema main, so a declared attribute without a Go field fails the SDK build. This adds the field to the internal player model.

Scope: internal model only. A public Underage accessor on types.Player is a follow-up.

Merge order

Merge oddin-gg/oddsfeedschema#22 first, then re-run CI here. The schema test compares against oddsfeedschema main in both directions, so until the schema lands this PR fails with sdk-only attribute /player_profile/player@underage — that is the expected red, not a bug. After the schema merges, main here fails the same way in reverse (schema-only attribute) until this PR merges, so land the two back to back.

Verification

• ODDSFEEDSCHEMA_DIR=<schema branch> go test -count=1 ./... — green; against ODDSFEEDSCHEMA_REF=main red on exactly the two underage slots (/player_profile/player, /competitor_profile/players/player), nothing else.
• Without the field, against the schema branch: red with schema-only attribute /player_profile/player@underage.
• gofmt, go vet clean.

🤖 Generated with Claude Code

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/oddin-gg/gosdk/internal/api/xml	0.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/oddin-gg/gosdk/internal/api/xml/player.go	0.00% (ø)	0	0	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.