What is the purpose of the change?

Fixes #16509.

MergeableClusterInvoker previously stored asynchronous results in a map keyed by service key. Multiple providers sharing the same service key could overwrite each other's results, silently losing data before merging.

What changed?

• Preserve every provider result in an ordered list instead of deduplicating by service key.
• Retain service keys for existing error diagnostics.
• Add a regression test with two providers sharing a service key but using different addresses.
• Verify that merged results contain data from both providers.

Validation

• RED: The regression test fails against the original implementation with the expected missing-data assertion.
• GREEN: Both MergeableClusterInvokerTest and MergeableClusterInvokerSameServiceKeyTest pass.
• Maven build, Spotless, and Checkstyle pass for the focused module build.
• Only two files changed.
• Upstream CI is pending.

Tested against Dubbo 3.3.7-SNAPSHOT.