Reject invalid YAML frontmatter in a single, self-identifying test

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A single malformed topic frontmatter file (e.g. display_name:zork missing the space after the colon) produced ~825 identical, non-specific invalid YAML: ... at line 3 column 18 failures, because metadata_for flunked on parse errors and the alias cross-check test re-parses every other topic — so one bad file flunked once per topic, never naming the culprit.

Changes

• Decouple parsing from assertion (test/test_helper.rb): metadata_for now only parses, returning nil on Psych::SyntaxError instead of calling flunk. This stops the cascade through tests that read other items' metadata.
• Self-identifying YAML check: new yaml_syntax_error_for returns a path-qualified message (invalid YAML in topics/zork/index.md: ...) or nil.
• Isolated per-item test: added it "has valid YAML frontmatter" to topics_test.rb and collections_test.rb, each parsing only its own index.md.
• Dedup: extracted a shared frontmatter_for helper used by both metadata_for and yaml_syntax_error_for.

Result

A broken zork/index.md now fails exactly one test, pointing straight at the file:

topics::zork topic#test_0001_has valid YAML frontmatter [test/topics_test.rb]:
invalid YAML in topics/zork/index.md: (<unknown>): mapping values are not allowed in this context at line 3 column 18

Other items' cross-checks treat an unparseable foreign file as having no aliases (its own YAML test already reports the failure), so no incidental noise.

[Copilot]

Pull request overview

Adds targeted YAML frontmatter validation while preventing cross-topic parse-error cascades.

Changes:

• Extracts shared frontmatter parsing.
• Adds path-qualified YAML syntax errors.
• Adds per-topic and per-collection syntax tests.

Show a summary per file

File	Description
test/test_helper.rb	Separates parsing from syntax reporting.
test/topics_test.rb	Adds topic YAML validation.
test/collections_test.rb	Adds collection YAML validation.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Medium