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.

pyproject.toml

@@ -39,6 +39,11 @@ dev = [
     "pytest>=8.0",
     "pytest-subtests>=0.13",
 ]
+# Amazon Bedrock support (AWS SigV4 auth + boto3). Optional so the core install
+# stays lean: pip install "tradingagents[bedrock]".
+bedrock = [
+    "langchain-aws>=1.5.0",
+]
 
 [project.scripts]
 tradingagents = "cli.main:app"