Backport 26.1: parser cleanup - #4456

Backport of #4442 to the 26.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:

• ./gradlew test --tests graphql.parser.StringValueParsingTest (15 tests)
• ./gradlew check -x test -x testng javadoc