Feature/add bedrock cost tracking support

[vladimirrotariu]

Summary

• Adds Bedrock cost-tracking support
• Preserves existing _bedrock provider string for compatibility

Resolves #1836
/claim #1836

Key Changes

Python SDK

• Extended LLMProvider enum with BEDROCK = "_bedrock"
• llm_usage_factory maps both:
  • LLMProvider.BEDROCK
  • Raw string "_bedrock"
    → to the Bedrock usage parser
• Bedrock decorator now uses LLMProvider.BEDROCK
• Unit test added: test_opik_usage_factory__bedrock_happyflow

Backend (Java)

• CostService maps:
  • "_bedrock", "bedrock_converse" → "bedrock" for pricing
• E2E test updates:
  • Includes Bedrock (normal + streaming) in SpansResourceTest#createAndGetCost

Docs

• No changes needed — existing Bedrock guides already use _bedrock

Testing

• Python: New unit test verifies Bedrock usage parsing
• Backend: Existing E2E cost test cover Bedrock models (including streaming), added cost computation