Simplify model parameter REST API

[jonahjung22]

Description:

• Fixes [model-parameters] Simplify the model parameters REST API #1464
• Remove unused UpdateModelParametersResponse type from frontend
• Remove redundant count field from GetModelParametersResponse
• Rename 'number' type to 'float' in ParameterSchema for clarity
• Add Pydantic models for type safety in backend

Code changes:

• UpdateModelParametersResponse was unused - frontend already has model_id, HTTP status codes handle errors
• GetModelParametersResponse.count is unnecessary - frontend can get array length
• ParameterSchema 'number' type was ambiguous vs 'integer' - now uses clear 'float' type
• Missing Pydantic validation for GetModelParametersResponse and ParameterSchema

User facing changes:

• No user-facing changes, internal API cleanup and improved type safety

[dlqqq]

@jonahjung22 Thank you Jonah! I left a couple of minor suggestions below.

After addressing the feedback, please also re-test this PR now that it is rebased onto main with #1473 merged.

I recommend running git config --global pull.rebase true if you haven't already. After running that, running git pull should sync your local branch with this remote.

[dlqqq]

The API looks great now, thank you for fixing it! 🎉