chore: Set default values for all documented optional env vars in example env files

Extend the previous commit's rule to every documented optional env var
in the example .env.example files: if the comment says "defaults to X",
the value is set to X inline and the comment is reworded to "Override...".

This covers AWS_DEFAULT_REGION in the bedrock example, which had the
same os.getenv(key, default) vs dotenv-loads-empty-string bug as the
LaunchDarkly config-key vars.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jsonbailey

getting_started/bedrock/converse/.env.example

@@ -5,8 +5,8 @@ LAUNCHDARKLY_SDK_KEY=
 AWS_ACCESS_KEY_ID=
 AWS_SECRET_ACCESS_KEY=
 
-# Optional - defaults to "us-east-1"
-AWS_DEFAULT_REGION=
+# Override to use a different AWS region
+AWS_DEFAULT_REGION=us-east-1
 
 # Override to use a different AI Config
 LAUNCHDARKLY_COMPLETION_KEY=sample-completion