fix: use AWS_REGION env var instead of hardcoded us-east-1 in dynamo_utils.py

[jyotsnamas]

Summary

• Fix 09-mcp-e2e/helpers/dynamo_utils.py which hardcodes region_name="us-east-1" for the DynamoDB client
• deploy.py creates the DynamoDB table in the session region (e.g., us-west-2), but the runtime agent code connects to us-east-1
• All DynamoDB tool calls fail with AccessDeniedException because the table doesn't exist in us-east-1

Root cause

# BROKEN (line 10):
def __init__(self, table_name="finance_tracker", region_name="us-east-1"):

# FIXED:
def __init__(self, table_name="finance_tracker", region_name=os.environ.get("AWS_REGION", os.environ.get("AWS_DEFAULT_REGION", "us-east-1"))):

Verification

Deployed 09-mcp-e2e/01-server-e2e to AgentCore Runtime in us-west-2 (with mcp pinned):

• Without fix: tools/call add_transaction → AccessDeniedException (table not in us-east-1)
• With fix: tools/call get_balance → Balance: $-42.50, Total Income: $0.00, Total Expenses: $42.50

Test plan

• Deploy 09-mcp-e2e/01-server-e2e with region fix → all finance tracker tools work
• tools/list returns all 5 tools
• tools/call add_transaction, get_balance return correct data
• resources/list and prompts/list work

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