[forwarder][lambda]: don't fail to parse on managed instances report log

[duncanista]

What does this PR do?

Updates how enhanced metrics are parsed in order to work with Lambda Managed Instances

Motivation

Failing to parse a report log from Lambda Managed Instances, causing other metrics to also not be sent

Encountered an error while trying to parse and submit enhanced metrics for log {'id': '39425407939120316110070836392653955405483598923294834689', 'timestamp': 1767896435215, 'message': '{"time":"2026-01-08T18:20:35.215Z","type":"platform.report","record":{"requestId":"4fe12301-721e-4428-b5b8-8cc01d87c1a9","metrics":{"durationMs":2.139},"spans":[{"name":"responseLatency","start":"2026-01-08T18:20:35.214Z","durationMs":0.621},{"name":"responseDuration","start":"2026-01-08T18:20:35.215Z","durationMs":0.061}],"status":"success"}}', 'aws': {'awslogs': {'logGroup': '/aws/lambda/lmi-dev-baseline-python314-1min-x86_64', 'logStream': '2026/01/08/lmi-dev-baseline-python314-1min-x86_64[$LATEST.PUBLISHED]b908c081675b44af9bb93b6610dcd370', 'owner': '425362996713'}, 'invoked_function_arn': 'arn:aws:lambda:us-east-1:425362996713:function:lmi-dev-dd-forwarder'}, 'lambda': {'arn': 'arn:aws:lambda:us-east-1:425362996713:function:lmi-dev-baseline-python314-1min-x86_64'}, 'ddsourcecategory': 'aws', 'ddtags': 'forwardername:lmi-dev-dd-forwarder,forwarder_version:5.1.1,dd_preserve_stack:true,aws_cloudformation_logical-id:lmidevbaselinepython3141minx8664logsd9310d2f,aws_cloudformation_stack-id:arn:aws:cloudformation:us-east-1:425362996713:stack/lmi-dev-baseline/628f0390-ecb9-11f0-83a3-0e124f123293,aws_cloudformation_stack-name:lmi-dev-baseline,env:none,aws_account:425362996713,aws_cloudformation_logical-id:lmidevbaselinepython3141minx8664ce8f6b40,aws_cloudformation_stack-id:arn:aws:cloudformation:us-east-1:425362996713:stack/lmi-dev-baseline/628f0390-ecb9-11f0-83a3-0e124f123293,aws_cloudformation_stack-name:lmi-dev-baseline,dd_preserve_stack:true,functionname:lmi-dev-baseline-python314-1min-x86_64,region:us-east-1,service:lmi-dev-baseline-python314-1min-x86_64', 'ddsource': 'lambda', 'service': 'lmi-dev-baseline-python314-1min-x86_64', 'host': 'arn:aws:lambda:us-east-1:425362996713:function:lmi-dev-baseline-python314-1min-x86_64'}: 'billedDurationMs'

SVLS-8273

Testing Guidelines

Added unit test

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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