Allow implicit aws credential setting for AnthropicBedrockChatCompletionClient (#6561)

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## Why are these changes needed?

Allows implicit AWS credential setting when using
AnthropicBedrockChatCompletionClient in an instance where you have
already logged into AWS with SSO and credentials are set as environment
variables.

## Related issue number

Closes #6560 

## Checks

- [ ] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [ ] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [ ] I've made sure all auto checks have passed.

Co-authored-by: Jack Gerrits <jackgerrits@users.noreply.github.com>

Author: GeorgeEfstathiadis

python/packages/autogen-ext/src/autogen_ext/models/anthropic/_anthropic_client.py

@@ -1193,11 +1193,15 @@ def __init__(self, **kwargs: Unpack[AnthropicBedrockClientConfiguration]):
         if bedrock_info is None:
             raise ValueError("bedrock_info is required for AnthropicBedrockChatCompletionClient")
 
-        # Handle bedrock_info as secretestr
+        # Handle bedrock_info
         aws_region = bedrock_info["aws_region"]
-        aws_access_key = bedrock_info["aws_access_key"]
-        aws_secret_key = bedrock_info["aws_secret_key"]
-        aws_session_token = bedrock_info["aws_session_token"]
+        aws_access_key: Optional[str] = None
+        aws_secret_key: Optional[str] = None
+        aws_session_token: Optional[str] = None
+        if all(key in bedrock_info for key in ("aws_access_key", "aws_secret_key", "aws_session_token")):
+            aws_access_key = bedrock_info["aws_access_key"]
+            aws_secret_key = bedrock_info["aws_secret_key"]
+            aws_session_token = bedrock_info["aws_session_token"]
 
         client = AsyncAnthropicBedrock(
             aws_access_key=aws_access_key,