fix(kvrocks2redis): translate FLUSHDB/FLUSHALL via DeleteRange (#2177)

[advisedy]

Fixes #2177.

Problem

FLUSHDB / FLUSHALL is implemented via DeleteRange on the metadata
CF, but the replication path did not propagate it:

• WriteBatchExtractor::DeleteRangeCF was a no-op, so the delete was
  silently dropped and never translated into a Redis command on replicas
  or kvrocks2redis.
• Database::FlushAll merged all namespaces into a single DeleteRange,
  which could not be mapped back to any specific namespace.

Fix

• FlushAll now emits one DeleteRange per namespace in the same batch.
• DeleteRangeCF translates those ranges into a per-namespace FLUSHDB,
  and rejects unrecognized ranges with a warning.

Tests

• Go integration test TestReplicationFlushDBAcrossNamespaces:
  reproduces the issue with repl-namespace-enabled=yes.
• C++ gtests in batch_extractor_test.cc for single-namespace FLUSHDB,
  multi-namespace FLUSHALL, and unrecognized ranges.

AI-assisted contribution

The tests were generated by an LLM. I reviewed all of them to make sure
they are correct and meaningful.

Scope

Only covers plain FLUSHDB / FLUSHALL. The MULTI/EXEC case
mentioned in #2177 is not handled here — I'm not sure what kind of
approach would fit best, and would appreciate reviewer input.

[Copilot]

Pull request overview

This PR fixes a replication/kvrocks2redis gap where FLUSHDB/FLUSHALL implemented as RocksDB DeleteRange on the metadata CF was previously dropped (no-op extraction) and thus never translated into equivalent Redis commands downstream—especially problematic when namespace replication is enabled.

Changes:

• Update Database::FlushAll to emit one metadata-DeleteRange per namespace (so ranges can be mapped back to namespaces).
• Implement WriteBatchExtractor::DeleteRangeCF translation of namespace-wide metadata DeleteRange into per-namespace FLUSHDB, with warnings for unrecognized ranges.
• Add Go integration coverage for cross-namespace replication and C++ unit tests for the extractor behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
tests/gocase/integration/replication/replication_test.go	Adds an integration test reproducing #2177 with repl-namespace-enabled=yes and verifying FLUSHALL replicates across namespaces.
tests/cppunit/batch_extractor_test.cc	Adds gtests validating DeleteRangeCF→FLUSHDB translation and handling of unsupported ranges.
src/storage/redis_db.cc	Changes FlushAll to emit per-namespace DeleteRange operations in a single write batch.
src/storage/batch_extractor.cc	Implements DeleteRangeCF handling for metadata range deletes, translating whole-namespace ranges into FLUSHDB.

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[advisedy]

Hi, @git-hulk , @PragmaTwice ,just friendly ping on this one, whenever you have some time. Thanks!