Fix unsafe row indexing and remove unnecessary Hash in stats endpoints

[adalpari]

Description

Addresses two issues flagged in PR #1200 review that also exist in the current codebase. These improvements enhance safety and code quality across all stats endpoints.

Changes

1. Fix unsafe row indexing in stats_visits.rs: The get_stats_data() function hard-coded row[0] to access the period field and used .unwrap() on position(). Now it safely looks up both the "period" and data field indices using position(), and uses .get() for bounds-safe access instead of direct indexing, which would panic on out-of-bounds access.

2. Remove unnecessary Hash derives: Removed Hash from 12 stats Period/Unit enums and 8 DataPoint/Response types across all stats files. Analysis showed none of these types are used as HashMap/HashSet keys anywhere in the codebase—Hash was blindly copied from other types. The only type genuinely requiring Hash is AttachmentMetadataKey (used as a HashMap key in support_tickets.rs), which was correctly retained.

Test plan

• All 1256 unit tests pass
• cargo build succeeds with no errors
• No new clippy warnings introduced (pre-existing warnings in other files remain)

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add to your Package.swift:

.package(url: "https://github.com/automattic/wordpress-rs", branch: "pr-build/1218")

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