Serialize APISIX standalone cache-miss dump with sync per cache key

[coderabbitai]

Summary

Serialize a cache-miss Backend::dump commit with Backend::sync for the same cache_key.

Rationale

A cache-miss dump can fetch an older standalone configuration while a sync completes. If dump then calls Cache::set_dump_result, it can replace the cache configuration and raw configuration with the older snapshot. A later unpinned sync can then build its document from stale state.

Concurrent sync requests for the same cache_key are outside the current operational model. This issue tracks the planned per-key locking needed to enforce that model and to prevent dump-versus-sync state replacement.

Required changes

• Add a per-key synchronization mechanism for rust/crates/adc-backend-apisix-standalone/src/backend.rs and the related cache flow.
• Serialize cache-miss Fetcher::dump execution and its cache commit with sync for the same cache_key, or use a generation check that rejects a fetched snapshot superseded by sync.
• Preserve independent operation for different cache keys.
• Add a coordinated regression test that verifies an older cache-miss dump cannot overwrite state written by sync.

Affected areas

• rust/crates/adc-backend-apisix-standalone/src/backend.rs
• rust/crates/adc-backend-apisix-standalone/src/cache.rs
• APISIX standalone backend cache and synchronization tests

Acceptance criteria

• A cache-miss dump cannot commit stale configuration after a sync for the same cache key completes.
• The cache configuration, raw configuration, and latest version remain mutually consistent.
• The regression test deterministically covers the dump-versus-sync ordering.

Backlinks

• Pull request: fix: rust standalone race conditon between dump and sync #570
• Review comment: fix: rust standalone race conditon between dump and sync #570 (comment)
• Requested by: @bzp2010