fix(models)!: deserialize model objects reflectively with gson - #473

Nothing ever constructed an ObjectMapper, so the 179 @JsonDeserialize
annotations and the two @JsonPOJOBuilder ones only described a deserialization
path this library never took. Gson does all the parsing.

BREAKING CHANGE: consumers who fed these model classes into their own
ObjectMapper got builder-based deserialization for free from those annotations
and now have to configure it themselves. Jackson remains on the test classpath
transitively via wiremock, but is no longer a compile dependency of the jar.

The "Default constructor." blocks say nothing, and a model builder's setter
javadoc repeats the field comment and the getter word for word. Neither is a
surface anyone browses: the builders are called by nothing since deserialization
became reflective.

Getter javadoc stays, since that is what a consumer reads in the published API
reference, and SpotifyApi and the requests package are untouched for the same
reason.

Deleting a comment on a public member makes doclint report it, so its missing
group is turned off. The javadoc CI step exists to catch dead references before
a tagged release attaches the docs, and all,-missing still fails the build on
those.

The Java version claimed 17+ while every profile compiles with --release 9, the
pinned version had drifted four release candidates behind, and the request
recipe still told agents to add a @JsonDeserialize annotation.

Adds a note on how model objects are deserialized now, since that changes what
adding a field to one costs.

ModelObjectGson delegates to the hand-written parser on Disallows, ExternalId
and ExternalUrl, so removing one of those overrides makes the inherited
reflective path call the adapter that calls it back, until the stack runs out.
PlaybackQueue's override is what returns an empty list for a missing "queue".

Disallows, ExternalUrl and PlaybackQueue were already covered by request tests.
ExternalId was not covered at all, and no model object holds one, so it is read
directly here. Each override also carries a note saying why it exists.

Beta added an externalIds field to Track and Album, which conflicted with the
reflective deserialization on this branch. The field, getter and toString entry
are kept; the hand-written parser branch beta added for it is dropped, since
reflection reads external_ids on its own now.

ExternalId is therefore no longer an orphan, so ModelObjectGson's adapter for it
is live and ModelObjectMappingTest asserts it through Album rather than alone.

The earlier pass spared any block whose @PARAM happened to contain a {@link} or
an <a href>, which split the setters on a distinction that carries no meaning:
"@PARAM copyrights {@link Copyright} objects." says no more than the signature.

Every link in the spared blocks is already on the matching getter, which keeps
its javadoc, so the published reference loses nothing. The javadoc run still
reports no warnings.

Nothing in the library, the tests or the examples referred to any of these.

PlaylistTrackPosition, ProductType and AlbumGroup lost their last caller when
10.0.0 dropped the getters that used them. Paging.JsonUtil and
PagingCursorbased.JsonUtil resolve their type argument to Paging<X> rather than
a Class, so instantiating either has always thrown; paging objects are built
through AbstractModelObject.JsonUtil.createModelObjectPaging, which takes the
element type from the enclosing JsonUtil.
IJsonUtil.createModelObjectArray(JsonArray, Class) had no callers left.

Also drops the @SuppressWarnings on the ExternalId and ExternalUrl parsers,
which no longer suppress anything.

BREAKING CHANGE: these types and methods are removed. They existed in 9.x, so
a consumer still referencing one will fail to compile against 10.0.0.

Adds the removed classes, the switch to reflective deserialization and the
dropped Jackson dependency, and the three getters that stopped returning null.

AGENTS.md now describes the example and README index step, what
ci/check-docs.py enforces, and the rule that a name removed from the public API
has to be recorded in a v9 column of MIGRATION.md so the guide keeps resolving.