Replace spoilered text with '[Spoiler]' in plain text fallback

[dozro]

Description

This pull request updates the plain text conversion logic to better handle spoilered text, ensuring that spoiler formatting is replaced with [Spoiler] in plain text fallbacks as per MSC4454.

Spoiler handling improvements:

• Spoilered text (delimited by ||) is now replaced with [Spoiler] in the plain text output, matching the MSC4454 specification.

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)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

No AI involved

[7w1]

[Spoiler]

lgtm

[github-actions]

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Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-715-sable.raspy-dream-bb1d.workers.dev	02bd5b8	pr-715	Thu, 23 Apr 2026 21:38:17 GMT