Serialization V2: fix docs and diagnostics drift; diagnose private nested message types

[Aaronontheweb]

Closes #8717. Part of #8675.

Changes

Docs text (docs/articles/serialization/source-generated-serialization.md):

• The "Not supported" paragraph, the "Limitations Today" section and the planned-work list no longer say an object collection element is unsupported. It has worked since Treat object elements inside collections the same as an object field #8528. They now say only an object dictionary key is rejected (AKKASG003).
• The Envelope Payloads section now says a collection element typed object is an envelope payload.

Diagnostic text:

• The cross-assembly variants of AKKASG007 and AKKASG015 said the generator cannot read a schema from a referenced assembly. That stopped being true with Support message types from referenced assemblies as nested fields and union members (F1) #8534 and Find protocol implementors and union members in referenced assemblies, and enforce placement (F3) #8537. The text now says the type has no [AkkaSerializable] schema the generator can read in that assembly, and names the fixes: annotate it there, register a formatter, or declare the type in this assembly.

New diagnostic (G-11):

• AKKASG045 (error), "Message type is not accessible to generated code". A private or protected [AkkaSerializable] type, or one nested inside such a type, used to fail with CS0122 in generated code and no AKKASG id. The generator now reports AKKASG045 at the type declaration and emits nothing for it. The message says to make the type and its containers internal or public.
• Added a docs paragraph and a row in the diagnostics reference table.

Tests (in AkkaSerializerGeneratorDiagnosticsSpec): private nested type, public type inside a private type, internal and public nested types stay clean, and the new cross-assembly AKKASG007 text.

Overlap with #8715

The generator PR for G-1 and G-3 (#8715) changes the supported-types table, the collections section and AKKASG007's text for TimeSpan and small scalars. This PR does not touch the table or any TimeSpan or scalar wording. The AKKASG007 change here is limited to the cross-assembly descriptor. The one-sentence edit in the "Not supported" paragraph sits just below the table, so a small textual merge may be needed.

Breaking changes

None