Improve TkMeasuredTest Coverage (related to #1408)

Refactor and extend TkMeasuredTest to improve coverage and robustness (related to #1408)

The current TkMeasuredTest verifies only the presence of a custom or default header. However, there are several aspects that could be improved:

1. Add negative/edge test cases:

   • What happens if a null or empty header is passed?
   • Does TkMeasured properly override an existing timing header?

2. Validate header value:

   • Currently, the tests only assert that the header exists, but not that it contains a valid numeric duration (in ms).

3. Improve assertion messages:

   • Current failure messages are generic and could be improved with more informative output (e.g. actual value of the response).

This refactor would help ensure better long-term stability and maintainability of the test.

I'm happy to prepare a PR for this if approved.