fix(tgbotapi): enforce rich date-time wire contract

[trakhimenok]

Production finding

Telegram Bot API 10.2 rejected a Preferans sendRichMessage because date_time_format: "relative" is not valid. The documented format is r|w?[dD]?[tT]?.

The existing validator only required a non-empty string, and its own fixture used the invalid word alias.

Fix

• validate the complete documented format grammar
• allow the documented empty format
• always emit required date_time_format and unix_time fields for RichTextDateTime, including valid zero values
• keep those variant-only fields absent from other rich-text types
• replace the invalid fixture and add exhaustive valid/invalid contract tests

Verification

• direct Telegram A/B probe reproduced the provider rejection for relative and accepted r through rich-message parsing
• GOWORK=off go test ./... -count=1
• gofmt -d
• git diff --check

This branch also contains the previously published Bot API 10.2 implementation from v0.15.5, bringing the release line back onto main.