feat: add Invalid Contributor verification

[ANISH-SR]

Summary of Changes

• Added a new equality check in process_closeaccount_device to verify that device.contributor_pk == contributor_account.key.
• If the keys do not match, the function now returns DoubleZeroError::InvalidContributorPubkey.
• This validation ensures that only the correct contributor is authorized to close a device account.

Testing Verification

• rust + fmt checks all green!

Fixes #2212

[juan-malbeclabs]

LGTM