fix(gateway): format-then-chunk in Telegram overflow split

[annguyenNous]

Fixes #43441

Problem

In Telegram bound/forum topics, final replies arrive showing raw Markdown markers literally. The root cause: _edit_overflow_split() chunks raw text first, then formats each chunk with MarkdownV2. The escaping inflates each chunk past the 4096 limit, Telegram rejects every formatted attempt, and the fallback delivers raw markers.

Fix

• When finalize=True: format content first, then chunk (matching send() order)
• Escape (1/N) chunk indicators for MarkdownV2
• Strip MarkdownV2 escapes in all plain-text fallback paths

Testing

• Syntax verified: ast.parse() passes
• finalize=False path unchanged

[alt-glitch]

Duplicate of #43458 — same fix (format with MarkdownV2 before chunking in _edit_overflow_split) for the same issue #43441. #43458 is the earliest open PR with this approach. See also #43463 (broader scope, also touches the stream-consumer cancellation path).

[liuhao1024]

Code Review: Single-chunk fallback re-formats already-formatted content

Thanks for the PR! The format-then-chunk approach is correct — formatting raw text before chunking prevents MarkdownV2 escapes from inflating past MAX_MESSAGE_LENGTH.

However, there's a double-formatting issue in the single-chunk fallback path. When finalize=True and len(chunks) <= 1, the code does:

# Line 18: content is formatted once
formatted = self.format_message(content)
chunks = self.truncate_message(formatted, ...)

# Line 36: single-chunk fallback re-formats the ORIGINAL content
chunks = [content if not finalize else self.format_message(content)]

self.format_message(content) is called twice on the same input. The first call (line 18) formats the text and the chunks list already contains the formatted result. The single-chunk fallback then reformats the original content instead of reusing the already-formatted formatted variable.

For MarkdownV2, format_message() escapes literal characters (e.g. * → \*). Double-formatting produces double-escaped output (e.g. \* → \\*), which renders with visible backslashes in Telegram.

Fix: Use formatted instead of re-formatting:

if len(chunks) <= 1:
    chunks = [formatted if finalize else content]

This correctly reuses the already-formatted text when finalize=True, and the raw text when finalize=False (unchanged behavior).

[austinpickett]

Closing as duplicate — #43458 (liuhao1024) covers the same format-before-chunk fix for the Telegram overflow split and is already under review.