fix: add typecheck to lambda wrapper

[avedmala]

What does this PR do?

• Checks if event is a dictionary before trying to call get() on it as it may be a list
• The case where it is a list is breaking instrumentation for customers
• Extended unit tests to catch this

Closes https://github.com/DataDog/datadog-lambda-python/issues/532

Motivation

Testing Guidelines

Additional Notes

Types of Changes

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

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog
• [ ] This PR passes the integration tests (ask a Datadog member to run the tests)

[avedmala]

/merge

[dd-devflow[bot]]

:steam_locomotive: MergeQueue: waiting for PR to be ready

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[dd-devflow[bot]]

:steam_locomotive: MergeQueue: pull request added to the queue

The median merge time in main is 0s.

Use /merge -c to cancel this operation!