AOT (6/7): remove UDP buffer pooling, keep the DNS provider configurable through a built-in table

[Aaronontheweb]

Changes

Removes two HOCON type-name hooks from Akka.IO, and keeps the one that is a real extension point.

UDP buffer pools: removed. DirectBufferPool was internal, off by default, and never asked for.

• UdpExt and UdpConnectedExt always construct DisabledBufferPool, reading only disabled-buffer-pool.buffer-size.
• Deleted: DirectBufferPool, the CreateBufferPool reflection helper in both extensions, and the buffer-pool / class / direct-buffer-pool config for udp and udp-connected.
• The public IBufferPool, BufferPoolInfo and BufferPoolAllocationException stay, now in IBufferPool.cs.
• UdpSettings.BufferPoolConfigPath is marked [Obsolete], matching TcpSettings.BufferPoolConfigPath.
• The dead akka.io.tcp buffer-pool config goes too (TcpExt never read it), along with the test-adapter and perf-spec lines that set it.

DNS provider: stays configurable, through the standard built-in table.

• provider-object keeps its meaning. InetAddressDnsProvider comes from a BuiltInDnsProviders table.
• Any other name loads by reflection while Akka.DynamicTypeLoading is on, and throws ConfigurationException (AkkaFeatures.NotBuiltIn) while it's off.
• The built-in provider's SimpleDnsManager and InetAddressDnsResolver are built with typed constructors. A custom provider keeps the Props.Create(Type, ...) path, which is [RequiresUnreferencedCode] and guarded so the trimmer drops it when the switch is off.
• A rooted Akka.AOT.App publish loses the five targeted IO/* trim warnings, and nothing else regresses.

Tests

• DnsProviderConfigSpec: DNS resolves end to end with the switch off; the built-in name's spellings resolve; a custom provider loads with the switch on and throws with it off.
• UdpDisabledBufferPoolSpec: the configured buffer-size is read.
• The UDP integration specs drop their direct-pool config. Three leak tests are removed; they could never fail, since DisabledBufferPool reports zero usage.

Ledger: two rows. The UDP row includes the migration step: move a custom buffer-size to disabled-buffer-pool.buffer-size, or datagrams larger than 512 bytes are cut off.

Also updates the coordinated-shutdown.md snippet range, which points into akka.conf.

Stack: PR 6 of 7 for AOT milestone 1. It isn't on the local boot path. Design and measurements: epic #7246.

Checklist

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue Epic: Full AOT Support for Akka.NET #7246
• Changes in public API reviewed: UdpSettings.BufferPoolConfigPath gains [Obsolete]; no other public API change.

[Aaronontheweb]

/azp run

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[Aaronontheweb]

Not quite there - we don't want to make buffer pools configurable at all for Akka.IO. That's an over-engineered feature we don't need to keep supporting.