feat: Update Java OpenFeature user ID mapping

[jonathannorris]

Summary

Adds support for userId field in OpenFeature Provider for Java Server SDK to match behavior across all DevCycle SDKs and OpenFeature providers.

Changes

• Enhanced DevCycleUser.fromEvaluationContext() to support userId as a fallback user ID source
• Implemented correct priority order: targetingKey → user_id → userId
• Added type safety validation for all user ID fields (must be strings)
• Ensured all potential user ID fields are excluded from custom data
• Added comprehensive test coverage for new functionality

Test Coverage

• Priority order validation across all user ID sources
• Custom data exclusion for all user ID fields
• Type safety with non-string values
• Fallback behavior when higher priority fields are missing

Compatibility

✅ Fully backward compatible - no breaking changes
✅ All existing tests continue to pass
✅ Matches behavior of web, iOS, and Android OpenFeature providers

Resolves consistency across DevCycle SDKs per internal discussion.

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Open Background Agent:
Web · Cursor

[Slack Thread](https://taplytics.slack.com/archives/C02DU5GTB6U/p1752872862815949%3Fthread_ts=1752872862.815949

[Copilot]

Pull Request Overview

This PR adds support for userId as a fallback user ID source in the OpenFeature Provider for Java Server SDK to maintain consistency across all DevCycle SDKs. The implementation follows a clear priority order: targetingKey → user_id → userId.

• Enhanced DevCycleUser.fromEvaluationContext() to support userId as the lowest priority fallback for user identification
• Added type safety validation ensuring all user ID fields must be strings
• Updated custom data exclusion logic to prevent user ID fields from appearing in custom data

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/com/devcycle/sdk/server/common/model/DevCycleUser.java	Implements userId fallback logic with type validation and updates custom data exclusion
src/test/java/com/devcycle/sdk/server/common/model/DevCycleUserTest.java	Adds comprehensive test coverage for priority order, type safety, and custom data exclusion