Bluesky threads reject cleared mentions as unsupported setting #475

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Description

Scheduling a Bluesky thread fails validation with mentions is not supported for Bluesky thread even after removing every written mention from the composer. Clearing the @ text is not enough: the Bluesky segment still keeps authored mention metadata (mentions: { version: 1, spans: [] }), and capability validation treats that object as an unsupported provider setting.

Steps to Reproduce

1. Go to a publication composer and create a thread with X and Bluesky destinations.
2. Insert a native Bluesky mention on one thread segment (or open an existing thread that already had one).
3. Remove the mention from the text so no @ remains on Bluesky.
4. Save / validate / schedule the publication.
5. See validation error: mentions is not supported for Bluesky thread.

Expected Behavior

Validation succeeds when there are no selected mentions. An explicit empty mentions object ({"version":1,"spans":[]}) should mean “no mentions”, not an unsupported Bluesky setting. Mentions should be validated by the dedicated mention path, not rejected as an unknown provider setting.

Actual Behavior

POST /api/v1/publications/{id}/validate returns unsupported_setting for field mentions on bluesky.thread. In the Network tab, the Bluesky segment still sends "mentions":{"version":1,"spans":[]} in settings after the visible mention text was cleared.

Screenshots

N/A (visible in Network + validation error in the composer).

Environment

• OpenPost version: 7.30.4
• Deployment method (binary/Docker/other): Docker
• Operating system: Linux
• Go version (if building from source): N/A
• Browser (if web UI issue): Firefox

Logs

mentions is not supported for Bluesky thread

Example Bluesky segment settings still present after clearing written mentions:

"settings": {
  "thread_gate": "everyone",
  "mentions": { "version": 1, "spans": [] }
}

Additional Context

Authored mentions are owned by posttext, not the Bluesky provider settings catalog (thread_gate, reply_gate, etc.). Empty spans still serialize as a non-empty object, so the unsupported-settings gate rejects them.

Proposed fix:

#474

Affected Platforms

• X (Twitter)
• Mastodon
• Bluesky
• LinkedIn
• Threads

(X side still works; the failure is reported on the Bluesky thread destination.)

Checklist

• I have searched for similar issues
• I have included all information requested above
• This is not a security vulnerability (if so, please email openpost+security@rgo.pt)