Render error caret for the last character of the parsing context

[fudianchn]

Problem

ParsingException.getMessage() renders a caret (^) under the error column, but guards it with column < context.length(). Because column is 1-indexed (the caret is drawn at position column - 1), the caret is dropped when the error is on the last character, i.e. column == context.length() (#625).

Fix

-            if (this.column >= 0 && this.column < this.context.length()) {
+            if (this.column >= 0 && this.column <= this.context.length()) {

<= renders the caret for the last character (at position column - 1) while still rejecting positions past the end. A short comment notes the 1-indexing so the boundary is not flipped back by mistake.

Verification

The caret branch from getMessage(), with context "hello" (length 5):

column	OLD (<) caret?	NEW (<=) caret?
5 (== length, last char)	no (bug)	yes, under the last char
3 (mid)	yes	yes (no regression)
6 (> length, past end)	no	no (still rejected)

Added ParsingExceptionTest:

• caretRenderedForLastColumn asserts the caret is rendered AND aligned under the last character (checks (length - 1) leading spaces, not just that a ^ appears).
• caretNotRenderedBeyondContext asserts no caret is rendered past the end.

./gradlew :core:checkstyleMain :core:checkstyleTest passes locally.

Closes #625