feat: support Bot API 10.1 (Rich Messages)

[aaliboyev]

Bot API 10.1 — Rich Messages

Adds the Rich Messages framework introduced in Bot API 10.1 (June 11, 2026). Follows the existing patterns in the library (the reaction.go discriminated-union style for the type unions, and the rawRequest wrapper style for methods).

Methods (methods.go / methods_params.go)

• sendRichMessage
• sendRichMessageDraft
• rich_message parameter added to editMessageText

Types (models/)

• Send side: InputRichMessage, InputRichMessageContent
• Receive side: RichMessage (+ rich_message field on Message); the RichBlock union (21 variants, incl. RichBlockThinking); the RichText union; plus RichBlockCaption, RichBlockListItem, RichBlockTableCell

Notes for review

• RichText is polymorphic — a value is a bare JSON string, an array of RichText, or a tagged object (25 variants, several recursively nesting text). It has a custom codec that dispatches on the first JSON byte; this is the one type that goes beyond the plain reaction.go shape.
• Value receiver on MarshalJSON for RichText/RichBlock (vs the pointer receiver in reaction.go). This is deliberate: RichText appears as a non-pointer struct field (Text RichText) throughout the variants, and encoding/json silently skips a pointer-receiver MarshalJSON on non-addressable values — so a value receiver is required for correct marshaling of nested rich text. RichBlock uses a value receiver too for consistency.
• RichBlockThinking is valid only in sendRichMessageDraft per the spec (ephemeral), and the field type is modeled accordingly.

Tests

Round-trip (unmarshal → marshal → assert) coverage for both unions — including the polymorphic string/array/object forms and recursive nesting — and request-shape tests for the two new methods. gofmt, go vet, and go test ./... are clean.

README "Supports Bot API version" and CHANGELOG bumped to 10.1.