fix(cache): stop the cache server from running its volume out of space (#1377)

Committing a cache upload copied every part into a new archive before deleting the parts, so a 5 GB save needed 10 GB free. Failed writes and assemblies left their partial bytes until the next stale sweep, up to an hour later, and every other save on the runner failed meanwhile. Nothing made room for an upload, so once the cache had filled its volume down to the free space floor, larger saves kept failing.

- Assemble in place, deleting each part once appended.
- Drop partial parts and archives on failure.
- Before a part would cross the floor, evict the least recently used entries.
- Reject uploads to reservations the sweep already deleted, which left files nothing removed.

_Written by Claude (Opus 5.5)._

Reviewed-on: https://gitea.com/gitea/runner/pulls/1377
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

internal/pkg/config/config.example.yaml

@@ -206,8 +206,8 @@ cache:
   # How the cache server discards entries, ignored when external_server or s3 is set since
   # those apply their own. Leave a setting out for its default; 0s or 0 turns the three
   # limits off. Sizes accept 10GB, 512mb, 1TiB or a plain byte count, binary either way.
-  # Whatever these allow, the cache still sheds entries to keep free space on its volume
-  # above health_check.min_free_disk_space_mb when health checks are enabled.
+  # Whatever these allow, the cache still sheds entries to keep free space on its volume, room
+  # for uploads included, above health_check.min_free_disk_space_mb when health checks are enabled.
   # Remove entries nothing has read or written within this window. Only last access counts.
   #retention: 168h
   # Cap one repository, removing its least recently accessed entries until it fits. An entry