[CI] Add cargo test --doc job to ci.yml

[davidmaronio]

Context

ci.yml's test job runs cargo test --features testutils but rustdoc examples (if any are added to public functions) are not exercised by a separate --doc pass, so broken doc examples wouldn't fail CI.

Acceptance criteria

• Behavior/gap described above is addressed or confirmed and documented
• Relevant unit test(s) added or updated
• cargo fmt --check and cargo clippy --all-targets --features testutils -- -D warnings pass

Files to touch

.github/workflows/ci.yml

Out of scope

• Unrelated refactors outside the files listed above
• Changes to unrelated contract functions not mentioned in this issue