What is the purpose of the change?

Refreshing a configuration in OVERRIDE_IF_ABSENT mode currently preserves top-level properties but overwrites already configured nested properties. For example, an explicitly disabled metrics aggregation becomes enabled when an external configuration sets dubbo.metrics.aggregation.enabled=true.

Reuse an existing nested configuration instance during refresh and check each property's current value against its actual owning object. Instantiate a nested configuration only when it is missing. This preserves configured nested values while continuing to fill absent values and apply external overrides in the other tested modes.

Add nine parameterized regression cases using real metrics configuration classes, including two nesting levels, an explicitly configured false, and previously absent nested objects. Also verify that a setter-only nested configuration remains supported when no getter exists. Before the fix, the two OVERRIDE_IF_ABSENT cases fail; after the fix, all ten cases pass.

Fixes #16513.

Validation

• Copying the same ten-case regression test into an isolated worktree at base commit ae809de2fc81b408194240965eb20bf27e6971df, with production sources unchanged, produces two assertion failures, both in OVERRIDE_IF_ABSENT mode. All ten pass with the fix.
• The latest local JaCoCo run passes 81 selected configuration-related tests in dubbo-common, including the ten regression/compatibility cases. The 48 selected dubbo-config-api tests and the 51-module dependency reactor previously passed; the follow-up commit changes tests only.
• Checkstyle reports zero violations; Spotless and git diff --check pass. The Unix line-ending check is skipped on Windows.
• The selected unit tests run with -DenableEmbeddedZookeeper=false; no external registry is required. Full-project and external-service integration tests have not been run locally.
• Both the initial upstream run and the latest upstream GitHub Actions run completed successfully for the submitted commits.

Coverage

The original Codecov partial line was the conditional lookup of the nested property's getter. The added setter-only compatibility test exercises the missing-getter path and verifies that the initialized nested object receives its external configuration value. Local JaCoCo confirms that all six executable lines changed in AbstractConfig.java are now fully covered, including both branches of that conditional (previously five of six, or 83.33%). The latest Codecov report confirms that all modified and coverable lines are covered by tests; the unit-test reports show 100% patch coverage.

Checklist

• Link the corresponding GitHub issue.
• Explain the problem, root cause, and resulting behavior.
• Add regression tests and verify that they fail before the production fix and pass afterward.
• Verify the upstream GitHub Actions results after submission.