Yaml: only treat % at the start of a line as a directive

[timtebeek]

• Fixes YamlParser not print idempotent when a comment before the first document contains % #8560

Problem

A comment before the first (implicit) document containing a % was parsed as a directive:

# 100%

key: value

printed back as

# 100%
key: value

• Parser.requirePrintEqualsInput then rejects the file and it becomes a ParseError — so any YAML whose header comment block contains a percent-encoded URL or prose like # 100% coverage drops out of the LST entirely. Introduced by Support for directives in Yaml #6529 (v8.72.0).

Fix

YamlParser.parseDirectivesFromPrefix scanned for % anywhere in the document prefix. It now applies the same "must be at the start of a line" guard that its sibling preScanDirectives already had.

While testing this I found a second, related bug: preScanDirectives treated a leading comment line as document content and skipped past everything up to the next ---, so a directive following a comment was never pre-scanned. It then fell back to reconstructing the directive from SnakeYAML's DocumentStartEvent, whose DumperOptions.Version enum has no 1.2 — so

# comment
%YAML 1.2
---
key: value

round-tripped as %YAML 1.1. preScanDirectives now accumulates a comment line into the pending directive prefix instead, keeping the directive's exact source text.

Tests

Four cases added to DirectiveTest, covering the variants from the issue (comment before a mapping, before a sequence, a comment starting with %) plus the directive-after-comment case above. All four fail on main.

[timtebeek]

Given the small size of the change, the readability, and test coverage, I'm fairly confident we can see this through and rely on asynchronous review if needed.