msggen: Add UnionField support to generators for oneOf schemas

[4xvgal]

Problem

msggen has a UnionField class (model.py:369-400) designed to represent oneOf JSON schemas, but no generator in the pipeline can actually render it. As a result, every schema field that uses oneOf is manually overridden to a single type (usually ArrayField or PrimitiveField), silently dropping variants.

This means the typed APIs (cln-rpc, gRPC, Python) cannot express what the C implementation and JSON schema both support. For example, invoice.exposeprivatechannels accepts true, ["1x1x3"], or "1x1x3" in JSON-RPC, but cln-rpc only exposes Option<Vec<ShortChannelId>>.

Related: ElementsProject/lightning#8961

Root Cause

1. UnionField.from_js() has a constructor bug — does not pass added/deprecated to __init__
2. CompositeField.from_js() has no oneOf detection — top-level oneOf fields hit the "type" not in ftype" warning and are skipped
3. No generator handles UnionField — all 6 generators (rust.py, proto.py, convert.py, unconvert.py, grpc2py.py, notification.py) only dispatch on PrimitiveField, ArrayField, EnumField, and CompositeField
4. Traversal utilities skip UnionField — patch.py, checks.py, and both gather_subfields functions don't recurse into union variants

Because of (2)-(4), every oneOf field requires a manual override in model.py:537-560, and every override is lossy.

Affected Fields (18 schemas use oneOf)

Override	Schema oneOf	What's Lost
Invoice.exposeprivatechannels	boolean | array[scid] | scid	boolean and single scid variants
Invoice.label (×5 methods)	string | integer	integer variant
Datastore.key (×4 methods)	array[string] | string	single string variant
Offer.amount	msat_or_any | currency	currency variant
SetConfig.val	string | integer | boolean	integer and boolean variants
Pay.exclude	array[scid_dir | pubkey]	pubkey variant (array-item oneOf)

Fix

1. model.py: Fix UnionField.from_js() constructor, add oneOf detection in CompositeField.from_js()
2. patch.py, checks.py: Add UnionField traversal
3. rust.py: Generate #[serde(untagged)] enums
4. proto.py: Generate oneof blocks with wrapper messages for array variants
5. convert.py / unconvert.py: Generate bidirectional From impls for union types
6. grpc2py.py: Generate WhichOneof()-based conversion
7. Remove 12 overrides that exist solely because oneOf was not handled