fix: stop Document.getPlainText repeating content past document length

[Yusufihsangorgel]

Closes #2704.

The problem

Document.getPlainText(index, len) repeats the document's content when len is
larger than the document:

final doc = Document()..insert(0, 'This is string 1');
doc.getPlainText(0, 50);
// "This is string 1\nThis is string 1\nThis is st..."
// expected: "This is string 1\n"

A blank document repeats its trailing newline the same way.

Root cause

Line._getPlainText (lib/src/document/nodes/line.dart) wrapped its whole body
in while (len0 > 0). That body already traverses the entire reachable document:
the inner while walks the current line's leaves, and the nextLine recursion
walks every following line. Once the document was shorter than len, the outer
loop ran again and re-processed the same nodes from the original offset,
padding the result by repeating the content. It is now a single pass.

Tests

Added a "getPlainText beyond document length" group to
test/document/document_test.dart, covering an oversized len and a blank
document. Existing tests are unaffected: every current getPlainText assertion
requests len equal to the available content, so none hit the repeat branch.

A note on how I verified

My machine is on Flutter 3.41 and this package's editor widgets need 3.44, so I
verified on the v11.5.0 tag, where Line._getPlainText is identical to master.
There the repro fails before the fix and passes after it, and the document suite
is green (17 tests). The change itself is a single while becoming if, so CI
on 3.44 is the final gate.