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

cmd/genapi/enums.go

@@ -271,32 +271,12 @@ func planUnifiedUnionEnums(api *spec.API) (map[string]enumDecl, map[string]strin
 			continue
 		}
 
-		// Build the candidate Go-name list. Priority order:
+		// Build the candidate Go-name list (shared with deriveDiscriminators
+		// in emitter.go so enum naming and decode dispatch never disagree
+		// on where to look). Priority order:
 		//  1. discriminator GoField from knownDiscriminators (resolved via JSONName);
 		//  2. "Type", "Status", "Source".
-		var candidateNames []string
-		seen := map[string]bool{}
-		add := func(name string) {
-			if name == "" || seen[name] {
-				return
-			}
-			seen[name] = true
-			candidateNames = append(candidateNames, name)
-		}
-		if ds, ok := knownDiscriminators[u.Name]; ok && ds.Field != "" {
-			// Resolve Go-name from the first variant whose field matches the JSON name.
-			for _, v := range variants {
-				for _, f := range v.Fields {
-					if f.JSONName == ds.Field {
-						add(f.Name)
-						break
-					}
-				}
-			}
-		}
-		for _, n := range []string{"Type", "Status", "Source"} {
-			add(n)
-		}
+		candidateNames := discriminatorFieldCandidates(u.Name, variants)
 
 		// Find the first candidate Go-name where every variant has a
 		// matching single-value string-enum field.