fix(telegram): handle message too long errors from HTML table expansion

[chenhg5]

Summary

• Handle "message is too long" error from Telegram when HTML table expansion exceeds 4096-char limit
• Reply/Send: chunk the HTML content into multiple messages using SplitMessageCodeFenceAware
• SendWithButtons: chunk with buttons attached only to the first message
• SendPreviewStart: fall back to plain text (preview messages shouldn't be chunked)
• Add sendChunked and sendChunkedWithButtons helper functions

Root Cause

The engine splits messages at 4000 chars based on Markdown length. But MarkdownToSimpleHTML() expands tables with column padding, causing the HTML output to exceed Telegram's actual 4096-char limit. Previously, this error was not handled, resulting in message drops.

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes
• Manual test needed: send a message with a wide Markdown table and verify it's chunked properly

Fixes #866

🤖 Generated with Claude Code

[chenhg5]

LGTM — targeted, correct fix for the Telegram 4096-char overflow edge case.

Root cause is correctly diagnosed: MarkdownToSimpleHTML() expands table columns with padding, so a message that fits Markdown's 4000-char budget can still exceed Telegram's actual 4096 HTML limit.

✅ sendChunked/sendChunkedWithButtons — reuse existing core.SplitMessageCodeFenceAware, which already respects code block boundaries; correct choice.

✅ sendChunkedWithButtons — buttons only on first chunk; avoids confusing "Submit" buttons appearing after the content they relate to.

✅ Preview messages → plain-text fallback (not chunked). Correct: preview handles are tied to a single message ID; chunking would break the edit-in-place model.

✅ Nested can't parse handler inside sendChunked — good defensive coding for malformed HTML in a chunk.

✅ Consistent errMsg local variable across all 4 send methods (small style improvement).

CI all green.