feat(genapi): derive discriminators and decode RichText alternates

The generator gated all union decoder emission behind knownDiscriminators, a
hand-curated map of 13 entries. Any sealed union absent from that map received
no decoder, so decoding must fail. Bot API v10 added RichBlock and RichText,
which were absent. Roughly 8.6 percent of Telegram updates failed with HTTP 400.

The generator now derives discriminators from the specification instead of a
fixed list. A gate check fails the build when a union resolves no discriminator.
Eleven unions gain decoders that never had one: BotCommandScope, InputMedia,
InputPaidMedia, InputPollMedia, InputPollOptionMedia, InputProfilePhoto,
InputRichBlock, InputStoryContent, PassportElementError, RichBlock and RichText.

RichText can also appear as a bare string or an array of RichText. The scraper
now extracts these alternate shapes into the intermediate representation. The
emitter adds RichTextPlain and RichTextSequence. The decoder dispatches on the
first JSON byte before it reads any discriminator.

Unions without alternates must regenerate byte for byte. The golden test proves
this.

Claude-Session: https://claude.ai/code/session_01Fu2wXqGUrfVBi5aLafKk8U

internal/spec/api.json

@@ -13717,6 +13717,14 @@
         "RichTextAnchorLink",
         "RichTextReference",
         "RichTextReferenceLink"
+      ],
+      "alternates": [
+        {
+          "shape": "string"
+        },
+        {
+          "shape": "array"
+        }
       ]
     },
     {

internal/spec/ir.go

@@ -24,6 +24,31 @@ type TypeDecl struct {
 	// OneOf, when non-empty, indicates this type is a union and lists the concrete variant type names.
 	// Variants are emitted as concrete structs implementing a sealed interface.
 	OneOf []string `json:"one_of,omitempty"`
+	// Alternates lists non-object wire shapes this union may also decode
+	// as, in addition to the object variants in OneOf. Only populated for
+	// unions whose doc declares them (Telegram's "can be either a String
+	// for plain text, an Array of X, or any of the following types:"
+	// phrasing) — as of Bot API v10 that's RichText alone, so every other
+	// type's serialised IR is unaffected by this field's existence.
+	Alternates []Alternate `json:"alternates,omitempty"`
+}
+
+// AlternateShape enumerates the non-object wire shapes a union's
+// Alternates field records.
+type AlternateShape string
+
+const (
+	// AlternateString: the union may decode from a bare JSON string.
+	AlternateString AlternateShape = "string"
+	// AlternateArray: the union may decode from a JSON array whose
+	// elements are themselves the same union type.
+	AlternateArray AlternateShape = "array"
+)
+
+// Alternate records one non-object shape a union type may appear as on
+// the wire, beyond its OneOf object variants.
+type Alternate struct {
+	Shape AlternateShape `json:"shape"`
 }
 
 // MethodDecl describes a Telegram API method.