fix: reject invalid savings pagination cursors

[brentthomas248]

Closes #1016

Summary:

• Add focused cursor-validation tests for active get_goals and archived get_archived_goals_page pagination.
• Cover empty indexes with non-zero cursors, cursor 0, past-highest IDs, u32::MAX, stale active/archived cursors, and valid monotonic traversal without duplicates.
• Fix the empty-index paths so non-zero cursors reject consistently instead of returning an empty page.

Verification:

• cargo test -p savings_goals cursor -- --nocapture
• cargo clippy -p savings_goals -- -D warnings
• git diff --check
• rustfmt --edition 2021 savings_goals/src/lib.rs savings_goals/src/test.rs

Notes:

• cargo test -p savings_goals -- --nocapture currently fails on the upstream baseline before this patch with unrelated existing failures: four goal-name validation tests and tests_safe_math::checked_add_returns_error_on_zero_amount.
• cargo fmt --all is currently blocked by an unrelated existing syntax error in reporting/src/lib.rs; the touched savings_goals files were formatted directly.