refactor(sessions): remove retired full-scan paths

[SaladDay]

Summary

• remove obsolete full-list session scanning, cache reconciliation, and progressive-preview paths superseded by paged manifests
• remove per-provider collectors and compatibility wrappers that were only reachable from the retired scan graph
• preserve the active bounded streaming, search, deletion, manifest, and downgrade-cleanup behavior
• keep full-provider cache inspection available only to tests so production code cannot regress to an O(N) cache load

Testing

• cargo fmt --check
• cargo check --lib --all-features
• cargo clippy --lib --all-features --no-deps -- -A clippy::reversed_empty_ranges
• cargo test --lib session_manager:: --all-features -- --test-threads=1 (192 passed)
• cargo test --lib --all-features -- --test-threads=1 (3,879 passed, 1 ignored)