Implement E2B provider

[dogancanbakir]

Description

Implement full E2B provider (currently a stub).

Use E2B's Code Interpreter API for AI-powered code execution.

Requirements

• Authenticate with E2B API key
• Create sandbox sessions
• Execute code with Code Interpreter
• Stream stdout/stderr
• File system operations
• Session cleanup
• Support for custom templates

References

• Current stub: internal/provider/e2b/e2b.go
• E2B docs: https://e2b.dev/docs
• E2B Go SDK: https://github.com/e2b-dev/e2b-go