[Test][TFLite] Add unit tests for PRELU

[fnhirwa]

This PR adds unit test coverage for PRELU activation
in the Relax TFLite frontend, as part of #18971

• Added unit test for PRELU and

Enabled converter to handle alpha broadcasting more cleanly across constant and expression-backed alpha inputs.

[gemini-code-assist]

Code Review

This pull request refactors the PReLU conversion in the TFLite frontend to handle alpha tensor broadcasting and adds new unit tests for structural and runtime verification. The review feedback highlights an opportunity to further optimize the implementation by using get_tensor_expr to unify the logic for constant and expression-backed tensors, thereby avoiding IR bloat caused by materializing large broadcasted constants.

[tlopex]

LGTM