Proposed changes (including videos or screenshots)

Stacked on #42975.

assertValidMessageBlocks checked the shape of blocks in code. The checks that a JSON Schema can express now live in the chat.sendMessage AJV schema:

• actions and context blocks must have elements, a list of objects.
• section fields, when present, must be a list of objects.
• input blocks must have an element object.

The code keeps only what a schema cannot express: the 100-point limit of linear_scale, its integer bounds wherever it is nested, and the nesting depth.

The accepted and rejected inputs do not change. Every unit case keeps its result; the unit spec now runs each case through the schema and the code, as the route does. For the shape errors the API now returns AJV's invalid-params message (e.g. must have required property 'elements') instead of Invalid blocks: …, so the e2e assertions match the new message. The status stays 400.

t is not touched, so video conference messages keep working.

Issue(s)

https://rocketchat.atlassian.net/browse/VLN-706

Steps to test or reproduce

chat.sendMessage with blocks: [{ type: 'actions' }] returns 400.

Further comments

The schema could also reject more fields (t, content, e2eMentions, location), but that would refuse requests that are accepted today, so it is left out of this change.