fix: rich messages follow-ups — reply_parameters, send latch, opt-in default

- Use reply_parameters per the sendRichMessage spec instead of the
  undocumented reply_to_message_id scalar (silently ignored -> reply
  anchor quietly dropped).
- Latch rich sends off after an endpoint-capability failure (old PTB /
  server without sendRichMessage) so every later reply doesn't pay a
  doomed extra roundtrip; per-message BadRequests do NOT latch.
- Default rich_messages to OFF (opt-in) while the day-old Bot API 10.1
  endpoint is validated live; revert the prompt-hint table guidance
  until the default flips on.
- Tests: reply_parameters shape, send-latch behavior, BadRequest
  non-latch; rich tests opt in explicitly via extra.

agent/prompt_builder.py

@@ -508,16 +508,13 @@ PLATFORM_HINTS = {
     ),
     "telegram": (
         "You are on a text messaging communication platform, Telegram. "
-        "Standard Markdown is automatically converted to Telegram formatting. "
+        "Standard markdown is automatically converted to Telegram format. "
         "Supported: **bold**, *italic*, ~~strikethrough~~, ||spoiler||, "
         "`inline code`, ```code blocks```, [links](url), and ## headers. "
-        "Telegram now supports rich Markdown, so when it improves clarity you "
-        "may use headings, tables (pipe `| col | col |` syntax), task lists "
-        "(`- [ ]` / `- [x]`), nested blockquotes, collapsible details, "
-        "footnotes/references, math/formulas (`$...$`, `$$...$$`), underline, "
-        "subscript/superscript, marked (highlighted) text, and anchors. Prefer "
-        "real Markdown tables and task lists over hand-built bullet substitutes "
-        "when presenting structured data. "
+        "Telegram has NO table syntax — prefer bullet lists or labeled "
+        "key: value pairs over pipe tables (any tables you do emit are "
+        "auto-rewritten into row-group bullets, which you can produce "
+        "directly for cleaner output). "
         "You can send media files natively: to deliver a file to the user, "
         "include MEDIA:/absolute/path/to/file in your response. Images "
         "(.png, .jpg, .webp) appear as photos, audio (.ogg) sends as voice "