fix(ts-sdk): check message.role instead of content for system messages

[ghdcksgml1]

Description

This PR fixes a bug in the message filtering logic where the code was incorrectly checking message.content instead of message.role to identify system messages.

This change aligns the TypeScript implementation with the Python implementation, ensuring that system messages are correctly skipped based on their role.

Reference:

• Python implementation: mem0/memory/main.py#L1428

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

I have verified that the logic now correctly checks the role property of the message object.

• Unit Test (If you ran existing tests, check this)
• Test Script (please provide)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Made sure Checks passed

[CLAassistant]

All committers have signed the CLA.