[libyaml]: document end fixes (9afa10a, fa1293a & 3694a4a)

[inteon]

Split from #97

Applies these upstream (libyaml) changes:

• yaml/libyaml@9afa10a (Always output document end before directive (YAML 1.2 compatibility) libyaml#160)
• yaml/libyaml@fa1293a (Output document end marker after open ended scalars libyaml#162)
• yaml/libyaml@3694a4a (Fix logic for document end before directive libyaml#174)

[Copilot]

Pull Request Overview

This PR applies three upstream changes from libyaml to improve document end tracking and handling in the YAML emitter. The changes replace a simple boolean flag with a more precise enumeration to track different types of document endings.

• Introduces a new yamlEndType enum to replace the boolean open_ended field
• Updates emitter logic to use the new enum values for better document end state tracking
• Removes outdated open_ended assignments and replaces them with appropriate end_type values

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
yamlh.go	Defines new yamlEndType enum and updates emitter struct to use end_type field
encode.go	Updates encoder finish method to use new enum value
emitterc.go	Replaces all open_ended usage with appropriate end_type enum values throughout emitter logic

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[ingydotnet]

I fixed the commit message issues from https://github.com/yaml/go-yaml/actions/runs/17546512705/job/49829110061
and force pushed back.

[ingydotnet]

I think this PR needs to wait until we have worked out our new options strategy.

[ingydotnet]

I think this PR needs to wait until we have worked out our new options strategy.

@inteon Now that we have Options in place, would you like to redo this one?