fix(server): flush trailing duplicate batch when last mutation is ignored

[cauchy1988]

Problem

pegasus_mutation_duplicator::duplicate() iterates the mutation set (sorted by timestamp), accumulating entries into a batch and flushing it when the batch-byte threshold is reached or the last mutation is seen. Mutations whose rpc code is RPC_RRDB_RRDB_DUPLICATE or RPC_RRDB_RRDB_BULK_LOAD are intentionally not duplicated.

The ignored-code path used continue, which also skipped the batch-flush check. So when the last mutation in the set was an ignored code, the in-progress batch of valid mutations was never moved to _inflights and was silently dropped. Because the caller has already advanced last_decree past the whole set, those mutations were never re-read and never duplicated — silent data loss (reported in #2284, with duplicate_log_batch_bytes > 0).

for (auto mut : muts) {
    batch_count++;
    ...
    if (gutil::ContainsKey(ingnored_rpc_code, rpc_code)) {
        ...
        continue;          // <-- skips the flush check below
    }
    ... add entry to batch ...
    if (batch_count == muts.size() || batch_bytes >= FLAGS_duplicate_log_batch_bytes || ...) {
        ... flush batch to _inflights ...   // <-- never reached if the last mutation was ignored
    }
}

Fix

1. Replace continue with if/else so the flush check always runs — the trailing batch is now flushed even when the last mutation is ignored.
2. Flush only when the batch is non-empty (an ignored mutation with an empty trailing batch is a no-op, preserving the all-ignored → cb(0) path covered by the existing duplicate_duplicate test).
3. Compute the batch's routing hash from the last entry in the batch (always a duplicatable code: PUT/REMOVE/MULTI_*), not the current (possibly ignored) mutation — get_hash_from_request would LOG_FATAL-abort on DUPLICATE/BULK_LOAD.

Testing

Added regression test duplicate_ignored_last_mutation: 3 valid PUTs followed by an ignored BULK_LOAD with the highest timestamp (iterated last). It fails before the fix (mail_box empty — the trailing PUT batch is dropped) and passes after.

Full duplicator suite passes (16/16):

[ RUN      ] pegasus_mutation_duplicator_test.duplicate_ignored_last_mutation/0
[       OK ] pegasus_mutation_duplicator_test.duplicate_ignored_last_mutation/0 (1 ms)
[ RUN      ] pegasus_mutation_duplicator_test.duplicate_ignored_last_mutation/1
[       OK ] pegasus_mutation_duplicator_test.duplicate_ignored_last_mutation/1 (2 ms)
...
[==========] 16 tests from 1 test suite ran. (6130 ms total)
[  PASSED  ] 16 tests.

Existing tests (duplicate, duplicate_failed, duplicate_isolated_hashkeys, duplicate_duplicate, get_hash_from_request, read_after_get_hash_key, create_duplicator) all still pass — no regression.

Closes #2284