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/model_catalog.py

@@ -193,6 +193,8 @@ MODEL_OPTIONS: ProviderModeOptions = {
     "kimi": _CUSTOM_ONLY,
     "groq": _CUSTOM_ONLY,
     "nvidia": _CUSTOM_ONLY,
+    # Bedrock model IDs / cross-region inference profile IDs are user-specified.
+    "bedrock": _CUSTOM_ONLY,
 }