chore: Track the latest v3 contract test release instead of a pinned alpha

[devin-ai-integration]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Follows launchdarkly/go-server-sdk#433.

Describe the solution you've provided

The FDv2 contract tests cloned sdk-test-harness, checked out the v3.0.0-alpha.6 tag and built it from source. That step is replaced with the shared launchdarkly/gh-actions/actions/contract-tests action using version: 'v3', which downloads the newest published v3.* release, so CI stays on the latest alpha without further pins. The Go setup step is no longer needed since the harness is no longer built locally.

Test parameters are preserved (-status-timeout=360, FDv2 suppressions, debug logging, stop-service-at-end, persistence tests off); the suppression path is now exported as SUPPRESSION_FILE_FDV2 alongside the existing v2 one.

Dropping the Go setup step also removed the incidental delay between the v2 run stopping the test service and the FDv2 run relaunching it, so the relaunch raced the old process for port 8000 and the harness never reached the service. The launch step now waits for port 8000 to stop answering before starting the new service.

Describe alternatives you've considered

Keeping the clone and resolving the newest tag with git tag --sort, which sorts prerelease suffixes unreliably (alpha.10 vs alpha.6).

Additional context

With the latest v3 alpha (v3.2.0-alpha.7) the FDv2 suite now fails 16 tests (anonymous-context attribute redaction, allAttributesPrivate with slash-prefixed attribute names, hooks environment ID). These are SDK behavior gaps newly covered by the harness, not a regression from this change — launchdarkly/python-server-sdk#492 fails the same tests. They need either SDK fixes or additions to test-supressions-fdv2.txt before this can merge.

How to test: run the workflows that pass run_fdv2_tests: true; the FDv2 contract test step should download the newest v3.* harness release and run to completion.

Link to Devin session: https://app.devin.ai/sessions/6409748db442482fa8ddc43061f0277d
Requested by: @jsonbailey

[devin-ai-integration]

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[devin-ai-integration]

@cursor review