TestKit: put the DotNetty batching override under akka.remote, where the transport reads it (re-land of #8546 with the RemoteConfigSpec fix)

[Aaronontheweb]

What changes

Re-lands #8546, which was merged without a CI run and reverted in #8560, and folds in the RemoteConfigSpec adjustment from #8559. This PR runs CI the normal way.

• src/core/Akka.TestKit/Internal/Reference.conf: the override remote.dot-netty.tcp.batching.enabled = false sat under akka.test, where nothing reads it, so it has been dead since it was written. It now sits under akka.remote, where DotNettyTransportSettings reads it. A test in TestKit_Config_Tests pins the path; it failed before the move and passes after.
• src/core/Akka.Remote.Tests/RemoteConfigSpec.cs: the batching fact reads the Akka.Remote reference config directly, since that is what its name says it checks. A sibling fact reads the running test system and asserts the TestKit override is applied, so a future misplacement of either key fails a test.

What this changes at runtime, so it can be judged rather than discovered

With the override live, DotNetty batching is off in every ActorSystem the TestKit creates, which includes every TestKit-based remoting test and every multi-node spec, since MultiNodeSpec pulls in the TestKit default config. Two consequences:

• The FlushConsolidationHandler leaves the pipeline for those tests. The shipped default, batching on, then has no end-to-end coverage in the test suites. If that matters, a handful of remoting specs should set batching.enabled = on explicitly.
• Any remoting test whose timing was tuned with batching silently on now runs without it. The comment on the override says batching was turned off to reduce flakiness in low-frequency tests, which was the intent all along, but it has never actually been exercised.

The first CI run of this PR is the first time the remoting suites run with the override in effect. Read its multi-node lanes with that in mind before merging.

How it was checked

Akka.Remote.Tests and Akka.TestKit.Tests build with warnings as errors. RemoteConfigSpec: 11 passed. TestKit_Config_Tests: passes. Test-only change. No ledger entry.