Builder: node types with specific implementations

Follow-up from PR #687 review feedback by @elijahbenizzy.

The visual builder currently has 9 node types (Action, Input, Result, LLM Call, API Call, Code, Streaming, Loop, Router) that are mostly visual/metadata. Each type should have a concrete implementation backing it, so the generated code produces functional Burr actions out of the box.

Scope:
- LLM Call: generate action with actual provider SDK call (OpenAI, Anthropic, etc.)
- API Call: generate action with HTTP client (httpx/requests)
- Code: inline Python execution
- Streaming: streaming action with proper `StreamingResultContainer`
- Loop/Router: proper transition logic with conditions

This was flagged as acceptable for the initial merge but important for the builder to be genuinely useful beyond prototyping.

Ref: https://github.com/apache/burr/pull/687#pullrequestreview-2795208280