pkl-core: Migrate nullness to JSpecify (#1601)

Replace pkl-core's local nullness annotations with JSpecify annotations.
Enable NullAway checking for pkl-core packages except org.pkl.core.ast
and org.pkl.core.stdlib.

Notable code changes:
- Add a dedicated late-init constructor to VmTyped
- Move VmExceptionBuilder's fallback message derivation from withCause()
to build()
- Split VmException rendering between builder-provided messages and
string-backed messages
- Initialize MessageTransport handlers with default throwing handlers
- Update JSON helper collection types to allow nullable values JSON
arrays and objects can contain JSON null,
so the Java Map/List element types need to model nullable elements
explicitly
- Make public command transform APIs accept nullable transformed values 
   Command transforms can produce null for optional/default handling,
so the BiFunction and options-map element types now model that
explicitly
- Make ExecutorSpiException accept nullable message and cause 
Existing call sites can pass nullable causes from Throwable.getCause()
- Remove JSR-305 semantics from `@LateInit`
   JSpecify does not support the same type-qualifier-nickname pattern,
so `@LateInit` is now documentation plus a NullAway
constructor-initialization exemption

Out of scope:
- NullAway checking of org.pkl.core.ast and org.pkl.core.stdlib
- IntelliJ warnings related to `@LateInit` fields
- Removing the JSR-305 dependency, since concurrency annotations are
still in use

pkl-core/src/test/kotlin/org/pkl/core/externalreader/ExternalReaderClient.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,13 +16,13 @@
 package org.pkl.core.externalreader
 
 import java.io.IOException
+import org.jspecify.annotations.Nullable
 import org.pkl.core.externalreader.ExternalReaderMessages.*
 import org.pkl.core.messaging.Message
 import org.pkl.core.messaging.MessageTransport
 import org.pkl.core.messaging.Messages
 import org.pkl.core.messaging.Messages.*
 import org.pkl.core.messaging.ProtocolException
-import org.pkl.core.util.Nullable
 
 /** An implementation of the client side of the external reader flow */
 class ExternalReaderClient(