fix(redis): atomic surrogate sets across instances, and Get on hash mappings

[silverbackdan]

Fixes darkweak/souin#870.

Problem

With a Redis store shared by several Souin instances, some cached responses end up unpurgeable by tag. There are two causes, depending on the module and version.

• redis (rueidis), and go-redis v0.0.19: neither implements SetStorer, so Souin's storeTag updates the comma-joined SURROGATE_<tag> string with a GET then a SET. It serialises that with a mutex that covers one process only, so instances sharing Redis overwrite each other's additions.
• go-redis v0.0.20: since feat(go-redis): store mappings as one hash field per variant #63, mappings are hashes, but Souin's SouinAPI.BulkDelete still reads them with Get(IDX_<key>). GET on a hash fails with WRONGTYPE, Get returns nothing and starts Reconnect(), and while it's reconnecting the Delete calls that follow are skipped. The variants and the mapping stay cached.
• go-redis AddToSet (smaller window): it reads a legacy string value, then DELs and SADDs in a separate transaction. Two writers migrating the same tag overwrite each other's members, even within one process.

Fix

• go-redis Get: on WRONGTYPE it reads the hash and returns it as an encoded core.StorageMapper. That's the format callers expect from a mapping key, and it's what BulkDelete decodes. A key that isn't a hash (a surrogate set, say) still returns nothing. Error replies no longer trigger Reconnect(), because the connection is fine.
• go-redis AddToSet: the legacy migration, SADD and lifetime extension now run as one Lua script. Behaviour is unchanged: legacy members are kept, a positive duration extends the lifetime but never shortens it, and a non-positive one leaves it alone. The lifetime is passed in milliseconds (PTTL/PEXPIRE).
• redis (rueidis): implements SetStorer (AddToSet with the same script, GetSet reading legacy strings or sets, WalkSets). Souin v1.7.9 already uses it when the storer supports it (perf(surrogate): store tags via native sets when the storer supports it souin#851).

No interface changes. The script touches a single key, so it works on Redis Cluster.

Release note

The */caddy modules tagged v0.0.20 still require their base modules at v0.0.19, so a standard xcaddy build --with github.com/darkweak/storages/go-redis/caddy ships go-redis v0.0.19. For this fix to reach xcaddy builds, the next release needs the make bump-version commit, as v0.0.19 had (#54).

Tests

Against the redis:8.4-alpine from compose.test.yml:

• go-redis: TestRedis_Sets_ConcurrentInstances has two storer instances add 40 members concurrently to a set that starts in the legacy string format, over 20 rounds. On main it fails on the first round with 21 of 42 members. TestRedis_Get_HashMapping runs BulkDelete's sequence against a hash mapping. On main, Get returns no entries.
• redis: TestRedis_Sets, TestRedis_Sets_LegacyStringMigration, TestRedis_WalkSets (ported from go-redis) and TestRedis_Sets_ConcurrentInstances. On main they fail because the storer doesn't implement SetStorer.
• go test -race passes for core, go-redis, redis and otter, with -count=3 for the two Redis modules.
• golangci-lint run --new-from-rev=main only reports exhaustruct on &core.KeyIndex{} and on the test clients' options literals. Those follow the existing code, which uses the same idioms.

End to end: two FrankenPHP instances with Souin v1.7.9 sharing one Redis, concurrent first requests for pages sharing one tag, then a purge of that tag through one instance. Pages still cached afterwards:

	60 pages ×3	10 pages ×3	200 pages
go-redis v0.0.19	3, 7, 5	0, 0, 2	–
go-redis v0.0.20	23, 47, 15	3, 3, 6	–
rueidis v0.0.19	8, 2, 10	0, 0, 0	–
go-redis, this branch	0, 0, 0	0, 0, 0	0
rueidis, this branch	0, 0, 0	0, 0, 0	0

On this branch every page is indexed in every run. Purges sent through either instance, or through a freshly restarted one, remove what the other stored, and purging a tag leaves tags that contain it indexed (darkweak/souin#867).

Not covered: Redis Cluster, and upgrading a live fleet where old and new binaries run side by side.

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