MavenResourcesExecution.copyOf() drops multiple fields

The `copyOf()` method at `src/main/java/org/apache/maven/shared/filtering/MavenResourcesExecution.java` lines 428–450 omits these fields:

- `flatten` (line 113) — controls flattened directory output
- `propertiesEncoding` (line 57) — encoding for .properties files
- `delimiters` (inherited from `AbstractMavenFilteringRequest`, line 75) — custom expression delimiter specifications
- `interpolatorCustomizer` (inherited from `AbstractMavenFilteringRequest`, line 84) — custom interpolator consumer

The default constructor calls `initDefaults()`, which sets delimiters to `["${*}", "@"]`. If a caller had configured custom delimiters or the other missing fields, the copy silently loses them. Any code path that creates a defensive copy (e.g., `mavenResourcesExecution == null ? new MavenResourcesExecution() : mavenResourcesExecution.copyOf()` in `BaseFilter` line 65) will produce an incomplete copy.

Read copyOf() in src/main/java/org/apache/maven/shared/filtering/MavenResourcesExecution.java lines 428–450, then inspect the listed fields and BaseFilter line 65 to understand the defensive-copy path. Ensure the copy retains flatten, propertiesEncoding, delimiters, and interpolatorCustomizer, and verify the behavior with the relevant Maven filtering tests.

Written by the indexing model from the issue text.