fix: MSC3389 relation JSON paths

[gamesguru]

Spawning from #913 (comment)

Didn't work for me. Kept failing. Found out, the test was wrong.

---

Summarized by gpt-5.6 luna low:

A path separator is the dot GJSON uses to move through nested JSON objects.

For example:

content.m.relates_to.rel_type

means:

{
  "content": {
    "m": {
      "relates_to": {
        "rel_type": "..."
      }
    }
  }
}

But Matrix actually uses a literal key named m.relates_to:

{
  "content": {
    "m.relates_to": {
      "rel_type": "..."
    }
  }
}

So the dot must be escaped:

content.m\.relates_to.rel_type

That tells GJSON to treat m.relates_to as one key.

---

Pull Request Checklist

• Pull request includes a sign off

[anoadragon453]

Thank you for this!