Add Java GSDK unit tests and CI workflow (#195)

* Initial plan

* Initial plan for Java GSDK unit tests

Co-authored-by: dgkanatsios <8256138+dgkanatsios@users.noreply.github.com>

* Add Java GSDK unit tests and CI workflow

Add 84 unit tests across 8 test classes covering:
- ConnectedPlayer data class
- SessionHostHeartbeatInfo getters/setters and JSON deserialization
- SessionConfig including ToMapAllStrings
- Operation enum Gson serialization with @SerializedName casings
- MaintenanceSchedule/MaintenanceEvent deserialization
- JsonFileConfiguration config file parsing and validation
- GameHostHealth/SessionHostStatus enum values
- GameserverSDK public API constant keys

Add .github/workflows/java-tests.yml CI workflow to run tests on PRs.

Co-authored-by: dgkanatsios <8256138+dgkanatsios@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dgkanatsios <8256138+dgkanatsios@users.noreply.github.com>

Author: Copilot