### What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: The file cache HTTP clear endpoint allowed callers to request synchronous clear with sync=true. The synchronous clear path is kept for unit tests and internal callers, but it is not safe to expose through the HTTP API. This change makes the HTTP op=clear endpoint always use async clear. The default behavior remains async, and explicit sync=true returns a warning message while still running async clear. Segment-specific HTTP clear also uses async remove instead of the synchronous remove path.

### Release note

The BE /api/file_cache?op=clear HTTP API no longer supports synchronous clearing. Requests with sync=true now run async clear and return a warning message.

### Check List (For Author)

- Test: Unit Test
    - DORIS_TOOLCHAIN=clang DISABLE_BE_JAVA_EXTENSIONS=ON ENABLE_INJECTION_POINT=ON ENABLE_CACHE_LOCK_DEBUG=0 ENABLE_PCH=0 sh run-be-ut.sh --run --filter=FileCacheActionTest.*
- Behavior changed: Yes. HTTP file cache clear no longer exposes synchronous clear.
- Does this need documentation: No

(cherry picked from commit 29d3a6f)
Picked-from: apache#64321