test(agent-core): add edge case unit tests for parseExecInput

[forumevi]

Summary

Adds edge-case unit test coverage for parseExecInput in client.test.ts.

Changes Included

• Added test coverage verifying that empty strings and empty JSON objects are handled as expected by parseExecInput.
• Added test cases covering invalid non-string and non-object inputs (e.g., JSON primitives like numbers and booleans).

Motivation

Ensures input parsing handles non-standard and boundary JSON values cleanly without unexpected runtime exceptions across agent workflows.