Fix ChainerBackend.delete_password to try all backends

[veeceey]

Summary

Fixes #697 where ChainerBackend.delete_password() was only trying the first backend instead of iterating through all backends in the chain.

The issue occurred when a password existed in a lower priority backend but not in the highest priority one. The delete_password method would try only the first (highest priority) backend, get a PasswordDeleteError, and fail immediately.

Changes

• Updated ChainerBackend.delete_password() to iterate through all backends, similar to how get_password() works
• Now catches PasswordDeleteError exceptions and continues trying other backends
• If all backends fail to delete the password, raises the last PasswordDeleteError encountered
• Added comprehensive test coverage for the new behavior

Testing

Added two new test cases:

1. test_delete_password_tries_all_backends - Verifies that deletion succeeds when password exists only in a lower priority backend
2. test_delete_password_raises_if_all_backends_fail - Verifies that appropriate error is raised when password doesn't exist in any backend

All existing tests continue to pass.

[veeceey]

Pushed a fix for the Windows CI mypy failure — the storage: dict = {} class-level type annotation was triggering mypy's annotation-unchecked note. Removed the type annotation since the __init__ method sets self.storage anyway.

The other two CI failures (Windows 3.9 pip install failure and pypy3.10 timeout/cancellation) appear to be transient infrastructure issues unrelated to these changes.