fix(bedrock_mantle): register xai.grok-4.3 for SigV4 Responses API routing

[syf2211]

Summary

Register bedrock_mantle/xai.grok-4.3 in the model cost map so Bedrock Mantle routes Responses API calls through BedrockMantleResponsesAPIConfig (SigV4-capable) instead of falling back to the bearer-only chat bridge.

Fixes #31196

Motivation

xai.grok-4.3 on Amazon Bedrock Mantle supports the native Responses API on the /openai/v1 path with SigV4/IAM auth, but LiteLLM had no model_cost entry for this model. As a result, mantle_supports_responses() returned False, get_provider_responses_api_config() returned None, and requests were forced onto the chat-completions bridge that only supports bearer tokens.

Changes

• Add bedrock_mantle/xai.grok-4.3 to model_prices_and_context_window.json and the backup JSON with:
  • use_openai_responses_path: true (AWS /openai/v1 wire path)
  • supported_endpoints: ["/v1/chat/completions", "/v1/responses"]
  • AWS pricing ($1.25/M input, $2.50/M output)
  • Reasoning/tool/vision capability flags from the AWS model card
• Add regression tests for Responses routing, pricing, registry membership, and mantle_supports_responses

Tests

pytest tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py::TestBedrockMantlePerModelResponsesURL::test_grok_4_3_uses_openai_responses_path \
  tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py::TestBedrockMantleResponsesPricing::test_grok_4_3_pricing_and_capabilities \
  tests/test_litellm/llms/bedrock_mantle/test_bedrock_mantle_responses_transformation.py::TestBedrockMantleResponsesPricing::test_models_registered -q

Result: 3 passed

Notes

• No routing/auth code changes needed — Bedrock Mantle onboarding is data-driven via model_cost.
• Tiered pricing above 200K tokens is not included (consistent with other bedrock_mantle/* entries today).

[greptile-apps]

Too many files changed for review. (500 files found, 100 file limit)

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[devin-ai-integration]

Superseded by rolling registry audit #37902, which reverified and absorbed the provider-documented metadata changes at field level