fix: normalize nested string-key tool arguments safely

[mikehostetler]

Summary

• normalize tool input keys recursively based on parameter schema types, including nested :map, {:map, ...}, {:list, :map}, and {:list, {:map, ...}}
• extend normalization for additional map-containing schema shapes: {:or, ...} and {:tuple, ...}
• keep atom safety by converting only schema-known or existing atoms (String.to_existing_atom/1), never creating new atoms
• remove remaining unsafe runtime String.to_atom/1 conversions in core runtime paths:
  • provider id inference in ReqLLM.Providers
  • model provider extraction in ReqLLM.Provider.Defaults
  • HTTP method parsing in streaming fixture context
  • Bedrock Converse role parsing
• preserve previous top-level behavior while fixing nested tool-call payloads from providers

Tests

• mix test --cover test/req_llm/tool_test.exs test/req_llm/schema_test.exs
• mix test test/req_llm/providers_test.exs test/req_llm/streaming/finch_client_test.exs test/req_llm/providers/amazon_bedrock/converse_test.exs test/req_llm/provider/defaults_test.exs
• mix test test/req_llm/tool_test.exs test/req_llm/schema_test.exs test/req_llm/providers_test.exs test/req_llm/streaming/finch_client_test.exs test/req_llm/providers/amazon_bedrock/converse_test.exs test/req_llm/provider/defaults_test.exs
• added regression tests for:
  • nested list/map normalization
  • unknown nested key atom safety
  • {:or, ...} map normalization
  • {:tuple, ...} map normalization
  • unknown provider-prefix decode path in defaults
  • method normalization in streaming fixture HTTP context
  • unknown Bedrock role fallback handling