fix: Retry-After in batch path response

[nbry]

This PR fixes the exception_factory usage in Vehicle.batch by using the headers from the individual path response rather than the top-level headers. This PR should resolve the missing "retry_after" field in the exceptions attached to the batch result object.

Summary of changes:

• Separate the handling of individual path responses into its own private static method for easier testing
• Use headers from the path response for path lambdas (instead of the top-level headers)
• Add an optional bool param to check the content-type in the headers in exception_factory. This check is not required for individual path responses in batch requests.

[smartcar-ci]

:tada: This PR is included in version 6.13.1 :tada:

The release is available on GitHub release

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