DOC-3526: TinyMCE AI now continues an earlier conversation when reopened

[kemister85]

Ticket: DOC-3526 (TINY-14487)

Site: Staging

Changes:

• Add an 8.7.0 TinyMCE AI bug fix release note: switching back to an earlier conversation in the Chat sidebar now restores its conversation ID so prompts continue that conversation.

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Pre-checks:

• Branch is correctly prefixed: feature/8.7.0/DOC-3526_TINY-14487
• modules/ROOT/nav.adoc has been updated (if applicable). Not applicable.
• Files have been included where required (if applicable).
• Files removed have been deleted, not just excluded from the build (if applicable). Not applicable.
• Files added for New product features include a release note entry. Not applicable (bug fix).
• Major or minor version changes have updated the supported-versions.adoc table. Not applicable.
• Build passes without console errors, warnings, or issues.

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Review:

• Documentation Team Lead has reviewed.