fix: Twitter/X thread finisher posting with HTML tags

[meysam81]

Previously, Twitter/X was posting thread finisher (message ending signature) with HTML

elements hardcoded into the message text. This was because the thread_finisher content was being used directly without HTML stripping.

This fix wraps the thread_finisher text with stripHtmlValidation() to remove HTML tags and convert them to plain text, consistent with how other platforms (Bluesky, Threads, Mastodon) handle this feature.

Changes:

• Added stripHtmlValidation() call for thread_finisher in X provider
• Now matches the implementation pattern used in Bluesky provider
• Ensures thread finisher posts plain text instead of HTML markup

The working set up is live and I've used it. Here's the proof of it working fine:

https://x.com/Meysamazing/status/1987512859727729141?s=20

fixes #1013

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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