OutOfMemoryError in postProcessCheckstyleReport() for large checkstyle-result.xml files

detected at: https://github.com/checkstyle/checkstyle/pull/19043#issuecomment-3986332943

`postProcessCheckstyleReport()` in diff.groovy reads the entire checkstyle-result.xml into a single Java String ([diff.groovy](https://github.com/checkstyle/contribution/blob/1392c4a4bbb95f53acb72518ba3a4173732e3e30/checkstyle-tester/diff.groovy#L708-L717))

```
def content = checkstyleResultFile.text
content = content.replace(oldPath, newPath)
checkstyleResultFile.text = content
```
For large projects (e.g., openjdk25 with full google_checks.xml), the XML file exceeds the hard JVM String size limit of 1,073,741,823 characters introduced in JDK 9 ([JDK-8190429](https://bugs.openjdk.org/browse/JDK-8190429)).

Error:[(link)](https://github.com/checkstyle/checkstyle/actions/runs/22553094943/job/65326166130#step:10:4346)
`java.lang.OutOfMemoryError: UTF16 String size is 1074003966, should be less than 1073741823`

Fix:
Replace the whole-file read with line-by-line streaming