fix: Raise TypeError for string exponents in EigenGate and add test

[luffy-orf]

This PR updates Cirq’s parameterized gate logic to disallow using strings as exponents (e.g., cirq.X**"text").

• If a string is passed as an exponent to any gate inheriting from EigenGate, a TypeError is now raised with a clear message.
• A test is added to ensure this behavior is enforced.

Changes -

• Added a type check in EigenGate.__pow__ to raise a TypeError if the exponent is a string.
• Added a test in common_gates_test.py to verify that cirq.X**"text" raises the expected error.

Closes - #2936
Solved Under unitaryhack