feat: global variables in Java

[fahad19]

What's done

This PR aligns the Featurevisor Java SDK with the latest JavaScript SDK behaviour and prepares the next breaking release as Java SDK v4.

It adds complete support for global variables, updates feature dependency evaluation, aligns sticky state and lifecycle modules, improves datafile change events, extends the OpenFeature provider, and substantially expands conformance coverage.

What changed

Global variables

• Added support for independently defined global variables.
• Added overloaded evaluateVariable and getVariable APIs using a single global variable key.
• Added overloaded Boolean, string, integer, double, array, object, and JSON helpers.
• Added getVariableKeys for global variable keys.
• Added getVariableEvaluations for aggregate global variable evaluation.
• Added support for caller defaults, sticky values, ordered and nested overrides, conditions, segments, and requiredFeatures.
• Included override keys and nested override paths in evaluation details.
• Added equivalent global variable support to child instances.

Example:

String message = f.getVariableString("welcomeMessage", context);
Object settings = f.getVariable("checkout设置", context);
Evaluation evaluation = f.evaluateVariable("checkout设置", context);

Required features

• Added the canonical requiredFeatures datafile model.
• Supported required feature checks using enabled state, variation, or both.
• Applied required feature checks to features, feature variables, global variables, and variable overrides.
• Ensured required feature evaluations pass through the complete module pipeline.
• Added the required_features_unmet evaluation reason and corresponding metadata.

Sticky features and variables

• Split sticky state into stickyFeatures and stickyVariables.
• Added setStickyFeatures and setStickyVariables.
• Added sticky_features_set and sticky_variables_set events.
• Added separate sticky feature and global variable support to child instances.
• Aligned sticky replacement and inheritance behaviour with the JavaScript SDK.

Aggregate evaluations

• Added getFeatureEvaluations for feature snapshots.
• Added getVariableEvaluations for global variable snapshots.
• Added support for selecting specific feature or global variable keys.
• Applied context and caller override options consistently.

Evaluation modules

• Added beforeEvaluation and afterEvaluation callbacks that work with both feature and global variable evaluations.
• Preserved feature specific before and after callbacks.
• Defined deterministic callback ordering.
• Ensured required feature checks use the complete module pipeline.
• Preserved transformed caller defaults through evaluation.

Datafile updates and events

• Added global variables to datafile parsing, merging, and replacement.
• Added dependency aware change detection across features, segments, required features, and global variables.
• Updated datafile_set events to report affected feature and global variable keys.
• Kept partial datafile update behaviour aligned with the JavaScript SDK.

OpenFeature provider

• Added global variable resolution through keys such as variable:welcomeMessage.
• Added configurable globalVariablePrefix.
• Added global variable override metadata and required feature reason mapping.
• Preserved ownership semantics when using an existing Featurevisor instance.
• Expanded provider tests for global variables and custom prefixes.
• Kept the OpenFeature provider as a separate Maven artifact.

CLI and integration

• Added global variable support to the test and benchmark commands.
• Added support for sticky global variables in project assertions.
• Added global variable evaluation metadata to CLI output.
• Expanded the shared SDK v3 conformance fixture and tests.
• Updated the CLI version to 4.0.0.

Breaking changes

• The Java SDK version is now 4.0.0.
• Sticky APIs now distinguish between features and global variables.
• Aggregate feature evaluations use getFeatureEvaluations.
• Global aggregate evaluations use getVariableEvaluations.
• Event names and public models have been aligned with the latest JavaScript SDK.
• Datafile dependency declarations use requiredFeatures.

Update the Maven dependency to:

<dependency>
  <groupId>com.featurevisor</groupId>
  <artifactId>featurevisor-java</artifactId>
  <version>4.0.0</version>
</dependency>

Applications using OpenFeature should use the matching provider:

<dependency>
  <groupId>com.featurevisor</groupId>
  <artifactId>featurevisor-openfeature</artifactId>
  <version>4.0.0</version>
</dependency>

Verification

• Java SDK unit tests.
• OpenFeature provider tests.
• CLI tests.
• Shared Featurevisor SDK v3 conformance suite.
• Global variable and requiredFeatures alignment tests.
• Module ordering and transformed default tests.
• Child instance tests.
• Datafile event dependency tests.
• Maven artifact boundary verification.
• Build and test coverage for all Maven modules.

Release

This PR prepares all Maven artifacts for tag:

v4.0.0

The tag workflow publishes the parent POM, Featurevisor Java SDK, and OpenFeature provider to GitHub 包.