chore: Refactor sfn->Lambda context injection code by merging two functions

[lym953]

What does this PR do?

1. Merge isSafeToModifyStepFunctionLambdaInvocation() into updateDefinitionForDefaultLambdaApiStep(), which is the only non-test caller
2. Delete the tests for the deleted function isSafeToModifyStepFunctionLambdaInvocation(). Add tests for updateDefinitionString() to test the deleted test cases.

Motivation

isSafeToModifyStepFunctionLambdaInvocation() only returns a true/false value, but we want to implement multiple instrumentation strategies based on the existing state of Parameters field, so we need to merge the state-checking logic into the caller function.

Testing Guidelines

Passed the existing and added tests

Additional Notes

Design doc: Fixing Step Function Instrumentation

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] 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

[lym953]

/merge

[dd-devflow[bot]]

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[dd-devflow[bot]]

:steam_locomotive: MergeQueue: pull request added to the queue

The median merge time in main is 2m.

Use /merge -c to cancel this operation!