Fix unused-import warnings under PG18 test builds

[SferaDev]

What

Compiling with the pg_test feature (e.g. make test PG_MAJOR=18) produced ~24 pre-existing warnings: 12 unused import and 12 never used (helper fns/consts), all inside test modules.

Why

These modules were gated #[cfg(any(test, feature = "pg_test"))], so they are compiled into the cdylib whenever the pg_test feature is on. But rustc strips #[test] functions outside --test builds, so in the cdylib pass the module body survives while every test is removed — leaving its imports and shared helpers dead. The warnings are version-independent; they just surfaced on PG18 because its target volume rebuilt from scratch.

Warnings fixed

unused import:

• src/copy.rs:3778 — super::*
• src/scan/exec/agg/extract.rs:170 — super::*
• src/scan/exec/agg/state.rs:711-717 — new_cd_set_int, StringArena, AggAccumulator/AggType/GroupKey*/hash_group_key*/keys_match, pgrx::pg_sys
• src/scan/exec/count_minmax.rs:885,887 — super::*, PG_EPOCH_OFFSET_*/encode_f*_to_i64
• src/scan/exec/decompress.rs:4330-4331 — super::*, std::cmp::Ordering
• src/scan/hook.rs:4766, src/scan/path.rs:2949 — super::*

never used: mk_key (blob_cache/storage.rs), ranges (copy.rs), SAMPLE_USEC (agg/extract.rs), assert_simple_matches_regex (agg/regex.rs), mk_segment (append_wire.rs), assert_like_eq/datum_from_i64/datum_from_f64 (batch_qual.rs), mk_filter/mk_cm (segments.rs), dict_col/lz4_col (text_col.rs)

Fix (cfg-gating choices)

• 13 modules containing only plain #[test] fns (copy.rs, blob_cache/storage.rs, scan/hook.rs, scan/path.rs, agg_wire.rs, append_wire.rs, batch_qual.rs, count_minmax.rs, decompress.rs, segments.rs, text_col.rs, agg/extract.rs, agg/state.rs): gate switched to plain #[cfg(test)] and the now-unneeded #[pgrx::pg_schema] dropped — matching existing precedent (compress.rs, catalog.rs, partition.rs, stats.rs, …). They never needed to live in the cdylib.
• src/scan/exec/agg/regex.rs mixes #[test] and #[pg_test], so the module keeps any(test, pg_test) (the #[pg_test] fns must ship in the cdylib); only the #[test]-only helper assert_simple_matches_regex is gated #[cfg(test)].
• Left untouched: modules with #[pg_test] fns (timeparse.rs, json_extract.rs, exec/mod.rs, agg/metadata.rs, agg/compact.rs, agg/parser.rs, functions/*, lib.rs) and agg/test_utils.rs + its mod declaration in agg/mod.rs, since test_utils is consumed by #[pg_test] modules and must stay in pg_test builds.

No production code paths changed — imports/cfg only.

Testing

• make build — clean, 0 warnings
• make clippy — 0 warnings
• make test (PG17) — 578 passed, 0 failed, 0 warnings
• make test PG_MAJOR=18 — 578 passed, 0 failed, 0 warnings

Integration suite not run (import/cfg-only change, no runtime code touched).