mdcode: reject non-list frontmatter tags

[amirhormati]

Problem

tags: in OKF frontmatter is meant to be a list. The Documents Layout does not
check that, and a JavaScript string is iterable, so a bare comma-separated
value is expanded into one Dataplex label per character.

tags: Stack Overflow, votes, posts, community in a real bundle file stored
this in Knowledge Catalog:

{" ": "true", ",": "true", "O": "true", "S": "true", "a": "true", "c": "true", ...}

No error, no warning. The entry looks fine locally and is corrupt in the
catalog.

Fix

parseMarkdown throws when tags is present and is not an array:

okf/bundles/stackoverflow/tables/votes.md: "tags" must be a list, got "Stack Overflow, votes, posts, community"

Coercing the string by splitting on commas is the other option. I did not take
it: a comma can legitimately appear inside a tag, so coercion silently invents
a different tag set than the author wrote. An error at parse time is cheap and
points at the file.

Two supporting details:

• parseMarkdown gained an optional source parameter so the message can name
  the file. Its only caller, loadEntry, passes the entry path.
• The outer condition moved from truthy to an explicit undefined/null check,
  so an empty-string tags is reported rather than skipped.

Verification

• tags: a, b, c throws with the file name and the offending value.
• A proper YAML list and frontmatter with no tags at all behave exactly as
  before.
• bun test ./tests/libts/scenarios.ts: 17 pass, 0 fail.

Ordering

Land after #293. The eight stackoverflow bundle files fixed there are the ones
that trip this guard, so merging this first makes that bundle fail to push.

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