[FFL-1318] Add openfeature-sdk gem to the test matrix

[Strech]

What does this PR do?

Adds openfeature-sdk to the test matrix and in addition add the gem stub due to absence of the original typespecs (but purely for used parts)

Motivation:

To reduce amount of noise in original PR

Change log entry

No.

Additional Notes:

Huge thanks to @marcotc

Closes #4962

How to test the change?

Not needed.

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-10 14:47:55

Comparing candidate commit 391695b in PR branch ffl-1318-add-openfeature-gem-into-test-matrix with baseline commit 6eff05b in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:profiling - sample timeline=false

• 🟥 throughput [-0.635op/s; -0.566op/s] or [-8.781%; -7.837%]

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 100.00%
• Total Coverage: 98.54% (+0.01%)

View detailed report

_{This comment will be updated automatically if new data arrives.
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