smartcontract: use validate_program_account! macro across processors

[elitegreg]

Summary

• Replace manual assert_eq!/assert! account validation with the validate_program_account! macro across 19 processor files in doublezero-serviceability
• Fix bug in link/resume.rs where link_account.is_writable was never validated despite the account being written to via try_acc_write
• Add consistent data_is_empty checks (via the macro) to accounts that previously lacked them

Details

The validate_program_account! macro consolidates four common checks (owner, non-empty, writable, PDA match) into a single call. Many processors were doing these checks manually and inconsistently. This PR standardizes them while preserving custom error handling where needed (e.g., AccessPassNotFound for empty accesspass accounts).

Files modified

• link/: accept, activate, closeaccount, delete, reject, resume, sethealth, suspend, update
• user/: activate, ban, check_access_pass, closeaccount, delete, reject, requestban, update
• device/: delete
• multicastgroup/: subscribe

Bug fix

link/resume.rs writes to link_account (line 83) but never checked is_writable. All other link processors that write validate writability. The macro now enforces this.

Testing Verification

• make rust-fmt passes
• make rust-lint passes (clippy clean)
• cargo test -p doublezero-serviceability — all tests pass