fix(theme-graph): report Liquid parse failures

[efegokdemir]

What are you adding in this PR?

When a Liquid file cannot be parsed, buildThemeGraph now records its URI and parser message in parseErrors, and serializeThemeGraph includes those diagnostics in the CLI JSON output. This prevents an incomplete graph from silently presenting the file as dependency-free.

Adds a regression test using a malformed Liquid section that references an asset; it verifies the parse failure is reported while graph construction remains non-throwing.

Fixes #1290

What did you learn?

An unparseable file must be distinguishable from a file with no dependencies.

Before you deploy

• I included a patch bump changeset
• Relevant tests, type-check, build, formatting, and git diff --check pass.