feat: add guardrails UI and backend support for agents

[Vansh150705]

Summary

Adds safety guardrails feature to the DocsGPT agent builder.

Changes

• Added guardrails_enabled, input_guardrails, output_guardrails fields to Agent TypeScript type
• Added guardrails toggle UI in agent builder Advanced section (master toggle + input/output sub-toggles)
• Added guardrails fields to API create/update routes
• Added guardrails columns to SQLAlchemy agents DB model
• Added guardrails support to AgentsRepository create/update methods
• Added Alembic migration 0007_agent_guardrails to add columns to agents table

How to test

1. Go to Agent Builder → Advanced section
2. Enable Guardrails toggle
3. Input and Output guardrail sub-toggles appear
4. Save/publish agent — settings persist correctly

Closes #1911

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