[hub] [backend] iron out pydantic versioning for templete, invocation params, content parts

[mikeldking]

Currently we don't have sufficient confidence that our current pydantic types sufficiently cover certain blocks of unstructured JSON - notably

• invocation params
• the ability to put other keys like citations inside of a content text part (test)

We also need to have some sort of documentation on how the pydantic versioning will work in the future if we ever need to introduce a breaking change to the schema of say a tool or a content part.

• [prompts] enforce valid jsonschema on output schema #6195
• [prompts] remove template type from input to create prompt mutations #6197
• [prompts] use discriminated union for content parts #6200