Nested directives combined with colon fences do not parse YAML blocks correctly

[iso2013]

What version of myst-parser are you using?

2.0.0

What version dependencies are you using?

docutils 0.20.1
sphinx 7.4.7

What operating system are you using?

Linux

Describe the Bug

When using this syntax:

::::{outer-directive}
:key: value
:::{inner-directive}
:::
::::

This warning message is displayed:
WARNING: 'outer-directive': Invalid options format (bad YAML) [myst.directive_parse]

This is because the directives.py file from MyST parser determines the end of the YAML block solely by looking at which lines begin with a colon.

I have a proposed fix which I will open a PR for momentarily.

Expected Behavior

No response

To Reproduce

No response