fix: update default model to command-r-plus-08-2024

[akhilesharora]

Summary

Update the default model from command-r-plus to command-r-plus-08-2024 because Cohere deprecated the old model name.

Root Cause

Cohere renamed the model command-r-plus to command-r-plus-08-2024. The old model name no longer exists in Cohere's API, causing fresh installs to fail when using the hardcoded default model.

Fix

Updated all references to the default model across the codebase:

• src/backend/config/default_agent.py
• src/backend/routers/utils.py
• src/backend/schemas/cohere_chat.py
• src/backend/tests/integration/conftest.py
• src/backend/tests/integration/routers/test_agent.py
• src/backend/tests/unit/factories/model.py
• src/interfaces/assistants_web/src/constants/conversation.ts
• src/interfaces/coral_web/src/constants.ts

Testing

Updated test fixtures and mocks to use the new model name. All existing tests pass with the updated model reference.

Closes #972

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