fix: default task root

[darklight3it]

Issue #, if available:

Description of changes:

Restore LAMBDA_TASK_ROOT fallback to process.cwd() for OCI runtime image customers.

The RIC 4.x refactoring introduced a hard requirement on the LAMBDA_TASK_ROOT environment variable, throwing a PlatformError when it's not set. This is a regression from RIC 3.x behavior where process.cwd() was used as the function root directory.

LAMBDA_TASK_ROOT is a restricted environment variable set by lambda worker environment to /var/task. Customers building custom runtime images via OCI cannot change this value, but were previously free to place their handler anywhere and rely on the working directory. The new strict check forces them to explicitly set LAMBDA_TASK_ROOT.

Changes

• Fall back to process.cwd() when LAMBDA_TASK_ROOT is not set, restoring RIC 3.x behavior
• Remove the now-unreachable PlatformError check for taskRoot (the fallback guarantees a value)
• Add unit test covering the fallback behavior

Testing

• Unit test verifying process.cwd() is used when LAMBDA_TASK_ROOT is unset
• Existing tests continue to pass

Target (OCI, Managed Runtime, both): both

#184

Checklist

• I have run npm install to generate the package-lock.json correctly and included it in the PR.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.