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.

api/unionparam_test.go

@@ -44,7 +44,7 @@ func TestSetMyCommands_BotCommandScope_NoPointerToInterface(t *testing.T) {
 	// is `BotCommandScope` (interface), not `*BotCommandScope`.
 	ok, err := SetMyCommands(context.Background(), bot, &SetMyCommandsParams{
 		Commands: []BotCommand{{Command: "start", Description: "begin"}},
-		Scope:    &BotCommandScopeAllPrivateChats{Type: "all_private_chats"},
+		Scope:    &BotCommandScopeAllPrivateChats{},
 	})
 	require.NoError(t, err)
 	require.True(t, ok)