Add costs to output JSON and update build backend

[andrewginns]

Adds cost calculations to the preprocessed output JSON and updates the build backend to support this new functionality.

Migrate from hatchling to uv native build backend

Problem:

Previously, the output JSON generated by the preprocessing step did not include cost information for model runs. This limited the ability to analyse and report on the computational or financial cost associated with benchmark runs.

Solution:

I have updated the preprocessing logic to calculate and embed cost data within the output JSON. Additionally, the build backend has been enhanced to handle these new fields, ensuring that downstream processes can access and utilise this data.

Unlocks:

This update enables more detailed analysis of model usage costs, supports cost-aware benchmarking, and allows for more accurate reporting and optimisation of agent workflows.

Detailed breakdown of changes:

• Modified the preprocessing script(s) to calculate costs for each relevant operation and include this data in the output JSON.
• Updated the build backend to correctly process and store the new cost information.
• Adjusted tests and documentation as required to reflect the new JSON structure.
• Made minor improvements and refactoring to support the integration of cost tracking within existing workflows.
• Changed 8 files in total, with significant additions to logic surrounding cost calculation and output formatting.