test: add Gherkin/testbed compliance tests to Java, Python, and Go

[aepfli]

Problem

Only Rust runs the official flagd Gherkin testbed specifications (testbed/gherkin/*.feature + testbed/flags/*.json). Java, Python, and Go all use ad-hoc inline flag configurations, meaning specification compliance is only verified in one language.

This creates a risk that host-language implementations silently deviate from the flagd spec, especially around edge cases covered by the testbed (zero-value flags, context enrichment, evaluator reuse via $ref, custom operators).

Shared Test Fixtures Available

testbed/gherkin/
├── evaluation.feature       # Basic flag types, zero values, defaults
├── targeting.feature        # $ref, fractional, sem_ver, starts/ends_with, edge cases
├── contextEnrichment.feature # $flagd.flagKey, $flagd.timestamp
└── metadata.feature         # Flag and flag-set metadata

testbed/flags/
├── testing-flags.json, zero-flags.json, custom-ops.json
├── evaluator-refs.json, edge-case-flags.json, metadata-flags.json

Proposal

Add test suites to each language that load the shared JSON fixtures and verify the same scenarios:

• Java: JUnit tests loading testbed/flags/*.json via updateState() and asserting evaluation results match expected values from the feature files
• Python: pytest tests doing the same via PyO3 FlagEvaluator
• Go: Go tests doing the same via wazero FlagEvaluator

This would immediately close the majority of cross-language test gaps identified in the gap analysis.

Priority

HIGH — this is the single biggest test coverage gap across the project.

[aepfli]

Consolidated Test Coverage Tracker

This issue now tracks all cross-language test coverage gaps. The following issues have been consolidated here since running Gherkin/testbed compliance tests would cover most of these scenarios:

• test: add custom operator tests to Java, Python, and Go #76 (custom operator tests) — covered by Gherkin testbed
• test: add zero-value flag tests to Java, Python, and Go #77 (zero-value flag tests) — covered by Gherkin testbed
• test: add $evaluator and $ref end-to-end tests to Java, Python, and Go #78 ($evaluator/$ref tests) — covered by Gherkin testbed
• test: add metadata merging tests to Java and Go bindings #80 (metadata merging tests) — covered by Gherkin testbed
• test: add object flag evaluation tests to Java, Python, and Go #82 (object flag tests) — covered by Gherkin testbed

Standalone issues that remain separate:

• test: add error handling tests to Python and Go bindings #79 (error handling tests for Python/Go) — not covered by Gherkin specs
• test: add concurrent access functional tests to Java bindings #81 (concurrent access functional tests for Java) — not covered by Gherkin specs

Priority: Low — benchmarks are the current focus.