ci: Use the v3 sdk-test-harness downloader for v3 contract tests

[kinyoklion]

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

None.

Describe the solution you've provided

Adds branch: 'v3' to the v3 launchdarkly/gh-actions/actions/contract-tests@contract-tests-v1 steps in java-server-sdk.yml and nightly-contract-tests.yml.

The v3 steps already pass version: 'v3', so the harness version is not pinned: run.sh resolves it to the newest v3.* release, prereleases included (the last main run used v3.2.0-alpha.12). But the action's branch input defaults to v2, so CI was downloading run.sh from the harness's v2 branch. With this change it uses run.sh from the v3 branch, which matches the harness version under test and what lib/sdk/server/contract-tests/Makefile already does locally.

Describe alternatives you've considered

Pinning a specific v3.x.y tag. Not done, because the request was to track the latest v3.

Additional context

The newest harness (v3.2.0-alpha.13) passes against main with the current suppressions. The two long-running FDv2 failures it finds without suppressions are fixed in a separate PR.

Link to Devin session: https://app.devin.ai/sessions/27eec66fcd28426ca483cf5a81b85704
Open in Devin Desktop: https://app.devin.ai/desktop/session/27eec66fcd28426ca483cf5a81b85704?variant=devin
Requested by: @kinyoklion

SDK-3263

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring