feat(llm): add Amazon Bedrock as a first-class provider

Bedrock uses the Converse API (langchain-aws) and the AWS credential chain, so
it has its own client like Anthropic/Google rather than the OpenAI-compatible
registry. langchain-aws is an optional dependency (pip install ".[bedrock]"),
lazy-imported with a clear install hint; importing the package never requires
it. The model name is a Bedrock model ID / inference profile ID.

tradingagents/llm_clients/validators.py

@@ -8,7 +8,7 @@ from .model_catalog import get_known_models
 # accepted without warning.
 _ANY_MODEL_PROVIDERS = (
     "ollama", "openrouter", "openai_compatible",
-    "mistral", "kimi", "groq", "nvidia",
+    "mistral", "kimi", "groq", "nvidia", "bedrock",
 )
 
 VALID_MODELS = {