feat: add on chain metrics for sign request payload version

[DSharifi]

Summary

Closes #2178

[claude]

PR title type suggestion: This PR is adding new user-facing functionality (on-chain metrics), so the type prefix should be feat:.

Suggested title: feat: add on-chain metrics for sign request payload version

[claude]

Code Review

One issue found:

• Unsafe arithmetic on u64 counters (crates/contract/src/lib.rs:214,218): The += 1 operations on sign_with_v1_payload_count and sign_with_v2_payload_count violate the project's "Use safe arithmetic methods" engineering standard. While overflow at u64::MAX is practically unreachable, the convention exists to avoid using primitive arithmetic operators. Consider using saturating_add:

  if request.deprecated_payload.is_some() {
      self.metrics.sign_with_v1_payload_count = 
          self.metrics.sign_with_v1_payload_count.saturating_add(1);
  }
  
  if request.payload_v2.is_some() {
      self.metrics.sign_with_v2_payload_count = 
          self.metrics.sign_with_v2_payload_count.saturating_add(1);
  }

Everything else looks good: state migration defaults counters to zero correctly, the DTO mapping is straightforward, tests follow the project's Given/When/Then convention, and the metrics() view method is properly exposed.

⚠️ (minor issue found)

[claude]

PR title type suggestion: This PR adds new on-chain metrics functionality to the contract, so the type prefix should be feat: instead of having no prefix.

Suggested title: feat: add on-chain metrics for sign request payload version

[claude]

PR title type suggestion: This PR adds new on-chain metrics tracking (payload version counts), which is a new user-facing capability. The type prefix should probably be feat: instead of chore:.

Suggested title: feat: add on-chain metrics for sign request payload version

[netrome]

I'm confused why we're adding these metrics in the contract. Shouldn't nodes keep track of this instead?

[netrome]

Edit: Discussed on slack. Since these are public methods we want to deprecate it's nice to have these metrics visible.

[gilcu3]

Thank you!