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/api_key_env.py

@@ -19,6 +19,8 @@ PROVIDER_API_KEY_ENV: dict[str, Optional[str]] = {
     "anthropic":  "ANTHROPIC_API_KEY",
     "google":     "GOOGLE_API_KEY",
     "azure":      "AZURE_OPENAI_API_KEY",
+    # Bedrock authenticates via the AWS credential chain, not a single key env.
+    "bedrock":    None,
     "xai":        "XAI_API_KEY",
     "deepseek":   "DEEPSEEK_API_KEY",
     # Dual-region providers each carry their own account; keys are not