Introduce LLM request builder

[kirbodevv]

Prompt generation, memory injection, schema generation, and message assembly are currently spread across multiple components.

Introduce a dedicated request builder layer for assembling all LLM requests consistently.

Tasks

• Create LlmRequestBuilder
• Move prompt assembly logic
• Move schema generation logic
• Inject memory context
• Assemble final message list

Example

builder.build(session, memory, input)