Add DB::KeyExists() public API to skip blob file reads for existence checks

[m-nagarajan]

Summary

The Java keyExists() was added as a JNI convenience that calls KeyMayExist + Get. For BlobDB stores, the Get step resolves blob indexes and reads potentially large (10KB–1MB+) blob values that are immediately discarded. This PR eliminates that waste by introducing a C++ DB::KeyExists() that both C++ and Java callers benefit from.

Details

Add a new DB::KeyExists() virtual method that checks key existence without resolving blob indexes. The existing Java keyExists() JNI method internally calls Get() which reads the full blob value only to discard it. The new API uses GetImpl with a non-null is_blob_index pointer, causing Version::Get to skip blob file I/O entirely.

• Add DB::KeyExists(ReadOptions, ColumnFamilyHandle*, Slice) returning Status::OK() or Status::NotFound()
• Add io_activity validation matching DB::Get() pattern
• Add StackableDB::KeyExists() override to prevent undefined behavior on TransactionDB/DBWithTTL
• Propagate is_blob_index in DBImplReadOnly::GetImpl and DBImplSecondary::GetImpl (all mem->Get, imm->Get, and Version::Get call sites)
• Simplify JNI key_exists_helper to delegate to DB::KeyExists() with a KeyMayExist bloom filter pre-check for fast rejection of absent keys
• Add C++ tests: blob-skip verification via BLOB_DB_BLOB_FILE_BYTES_READ statistics, basic existence, column families, merge operands, invalid io_activity, read-only DB
• Add Java tests: BlobDB correctness, blob-skip verification via PerfContext, column families, ReadOptions variant, memtable path, DirectByteBuffer

Related: #12921 reports keyExists false negatives from KeyMayExist. The C++ DB::KeyExists() avoids this by calling GetImpl directly without KeyMayExist. The JNI layer retains KeyMayExist as a fast-path (accepted trade-off for backward compatibility).

Testing

• db_blob_basic_test: 6 new KeyExists* test cases
• KeyExistsBlobTest.java: 6 new Java test cases including PerfContext-based blob-read verification
• Both test suites verify that keyExists produces zero blob reads while get() on the same key produces non-zero blob reads (control)