refactor(api): auto-inject discriminator value via generated MarshalJSON

Sealed-interface union variants now hardcode their wire discriminator
inside a generated MarshalJSON method instead of forcing callers to set
the field on every struct literal. Drops a class of silent-rejection
bugs where a typo in the discriminator slipped past the type checker
and through to Telegram, which then rejected the request with no
Go-side signal.

The discriminator field stays exported so incoming-message decoding,
type switches and debugging still see it. MarshalJSON wraps via a
function-local type alias and emits an outer field with the same json
tag; encoding/json (and goccy/go-json) resolve the outer field as the
shallower one and override whatever the caller wrote.

99 variants get MarshalJSON. 7 are skipped because their unions
dispatch structurally rather than by a string field: Message and
InaccessibleMessage (MaybeInaccessibleMessage, dispatched on date),
and the InputMessageContent family (InputTextMessageContent,
InputLocationMessageContent, InputVenueMessageContent,
InputContactMessageContent, InputInvoiceMessageContent — Telegram
identifies these by the presence of message_text / latitude /
phone_number / title etc.).

Discriminator extraction lives in the emitter (cmd/genapi/emitter.go).
Resolution: knownDiscriminators reverse-lookup for the 13 auto-decode
unions, then doc-string analysis ("must be X" / "always “X”")
of the variant's first required string field for marker-only unions
(BotCommandScope, InputMedia, InputPaidMedia, InputProfilePhoto,
InputStoryContent, InputPollMedia, InputPollOptionMedia,
InlineQueryResult, PassportElementError). Variants the emitter cannot
resolve a discriminator for are skipped silently rather than emitting
broken code.

Internal call-site cleanups: 4 manual discriminator assignments
removed (api/unionparam_test.go,
dispatch/filters/message/message_test.go, examples/inline/main.go ×2).
Regression tests added in api/marshaljson_variants_test.go covering
type-keyed variants, source-keyed variants, the override-user-typo
guarantee, round-trip preservation through UnmarshalChatMember, the
no-discriminator InputMessageContent path, and ride-along of
non-discriminator fields.

regen-from-fixture is deterministic across two consecutive runs;
go test -race / go vet / staticcheck all clean.

cmd/genapi/types.tmpl

@@ -84,6 +84,23 @@ func UnmarshalMaybeInaccessibleMessage(data []byte) (MaybeInaccessibleMessage, e
 type {{.Name}} struct {
 {{range .Fields}}{{docComment .Doc}}{{goField $td.Name .}}
 {{end}}}
+{{if variantHasDisc .Name}}
+// MarshalJSON encodes {{.Name}} with the discriminator field
+// "{{variantDiscField .Name}}" forced to {{printf "%q" (variantDiscValue .Name)}}.
+// The hardcoded value frees callers from setting {{variantDiscGoField .Name}} by hand —
+// any user-supplied value on the struct literal is overridden so a typo
+// can't slip through to Telegram.
+func (v *{{.Name}}) MarshalJSON() ([]byte, error) {
+	type alias {{.Name}}
+	return json.Marshal(&struct {
+		{{variantDiscGoField .Name}} string `json:"{{variantDiscField .Name}}"`
+		*alias
+	}{
+		{{variantDiscGoField .Name}}: {{printf "%q" (variantDiscValue .Name)}},
+		alias: (*alias)(v),
+	})
+}
+{{end}}
 {{$unionFields := unionFields .}}{{if $unionFields}}
 // UnmarshalJSON decodes {{.Name}} by dispatching union-typed fields
 // ({{range $i, $u := $unionFields}}{{if $i}}, {{end}}{{$u.Field.Name}}{{end}}) through their concrete UnmarshalXxx helpers.