refactor(backend): Centralize Block Cost into a Single File

[majdyz]

Currently, the cost for LLM blocks is defined separately (in llm.py) from the rest of the block cost (in credit.py).
We need a single place to configure these.

Changes 🏗️

• Moved blockcost-related models to cost.py.
• Merged LLM cost and the rest of the block cost to block_cost_config.py.

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
  • ...

Example test plan

• Create from scratch and execute an agent with at least 3 blocks
• Import an agent from file upload, and confirm it executes correctly
• Upload agent to marketplace
• Import an agent from marketplace and confirm it executes correctly
• Edit an agent from monitor, and confirm it executes correctly

For configuration changes:

• .env.example is updated or already compatible with my changes
• docker-compose.yml is updated or already compatible with my changes
• I have included a list of my configuration changes in the PR description (under Changes)

Examples of configuration changes

• Changing ports
• Adding new services that need to communicate with each other
• Secrets or environment variable changes
• New or infrastructure changes such as databases

[qodo-merge-pro]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Code Duplication The MODEL_COST dictionary duplicates the model list from MODEL_METADATA. Consider deriving MODEL_COST from MODEL_METADATA to avoid maintaining two separate lists. Default Cost Configuration The default cost configuration using GPT4O model should be validated to ensure it's the intended default choice for all unspecified cases.

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