test: expand coverage for edge cases identified in quality review

[larkly]

Description

The quality review identified four scenarios not covered by the current test suite.

Missing tests

1. Multiple root-level nodes — two * Node lines produce two sibling nodes directly under <map>; no test verifies this is handled correctly.

2. Hyperlink-only node roundtrip — [[http://example.com]] (no label) should survive a .mm → .puml → .mm roundtrip with the URI intact.

3. @startmindmap with title text — @startmindmap My Title should not crash the converter (behaviour may change once feat: preserve @startmindmap title text in conversion #16 is resolved).

4. Multiline continuation with leading whitespace — documents current behaviour and will need updating when bug: multiline continuation lines preserve leading whitespace unexpectedly #14 is fixed.