Unify common helper functions in rust transpiler passes

[ShellyGarion]

There are some helper functions:

• average_gate_fidelity_below_tol
• is_angle_close_to_multiple_of_pi_k

and a constant MINIMUM_TOL = 1e-12

that are used in several transpiler passes in rust:

• commutative_optimization.rs
• remove_identity_equiv.rs
• litinski_transformation.rs
• substitute_pi4_rotation.rs

These functions and constant are moved to a new common.rs file for a better code organization.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code: Claude Sonnet 4.6

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core