[Contracts] get_platform_stats always returns stats_are_partial=false and scanned_up_to=total_campaigns

[davidmaronio]

Context

Since counters were made O(1) (issue #411), get_platform_stats hardcodes stats_are_partial to false and scanned_up_to to total_campaigns every call. These fields can now never differ, so PlatformStats carries dead state that should either be removed or documented as retained for API compatibility.

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

src/queries.rs

Out of scope

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