fix(collector): exclude Self-receiver types only at receiver position, keep them as params elsewhere

[monchin]

Description

In last fix (b17dff2), I missed a situation that Py<Self> is not the first param, e.g.:

#[pymethod]
fn compare(&self, other: Py<Self>) -> bool {
    return true;
}

rylai would swallow other in last commit. So it should be fixed.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / chore

Checklist

• Code follows the project style (cargo fmt, cargo clippy).
• Self-review done.
• Comments and docs are in English.
• Pre-commit checks pass (prek run --all-files or equivalent).
• CI passes (build, test, lint on Ubuntu, Windows, macOS).