Backport of #4442 to the 25.x maintenance branch.

The original commit applies without changes.

Summary:

• makes block string indentation removal linear by retaining an index window over the split lines
• avoids repeated removal from the front of an array-backed list
• adds focused coverage for large runs of leading and trailing blank lines

Testing:

• RELEASE_VERSION=local-test ./gradlew test -x testWithJava11 -x testWithJava17 --tests graphql.parser.StringValueParsingTest (15 tests on Java 21)