Add datetime to default system prompts

[Adityai1411]

Summary

• add {datetime} to the default chat and agent system prompts
• expand variables for agent fallback prompts so {datetime} is resolved at request time
• add focused regression coverage for datetime variable expansion and default agent prompt expansion

Closes #6014

Tests

• npx --yes jest server/__tests__/models/systemPromptVariables.test.js server/__tests__/utils/agents/defaults.test.js --runInBand
• cd server && npx eslint models/systemSettings.js utils/agents/aibitat/providers/ai-provider.js