test: add edge case tests for embed expansion functionality

[swaploard]

Description

Fixes # (#71)Adds embed expansion edge-case tests TestEmbedExpansionEdgeCases in tests/test_synapse.py covers cycles, missing targets, and happy-path for both {{embed [[Page]]}} and {{embed ((uuid))}}

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update

🛡️ Sovereign Developer Checklist

• I have read CONTRIBUTING.md (or picked a task from docs/GOOD_FIRST_ISSUES.md).
• I have executed make all locally, and all linters (Ruff), type-checkers (Mypy), and tests (Pytest) pass successfully.
• I have added/updated tests for my changes (if applicable).
• I have updated the relevant documentation (README.md, docs/, CONTRIBUTING.md as needed).
• I updated CHANGELOG.md under [Unreleased] for user-visible behavior changes.
• My code follows the project's typing conventions (explicit hints on new functions; Pydantic strict=True on domain models).

Screenshots / CLI Output (if applicable)

[MarcoPorcellato]

Hi @swaploard and thanks for your PR!

Welcome to the "Logseq Matryca Parser".

I will evaluate your contribution and reply back to you,
Marco