feat(telegram): expose parseMode config option

[Koshkoshinsk]

Problem

Since #245, the Telegram adapter hardcodes parse_mode: "Markdown" whenever the caller passes { markdown: ... }. Telegram considers legacy Markdown a deprecated mode (see Bot API formatting options) and it has no escape support, so callers can hit Bad Request: can't parse entities with no way to recover other than dropping the message permanently.

For example, an agent reply containing routine markdown like:

- **Trigger required?** (does every message need the trigger…)
- **Folder** name (e.g. `telegram_family-chat`)

is rejected by Telegram, and there is currently no way for the caller to pick a different parse mode or disable parsing entirely.

Change

Adds an optional parseMode field to TelegramAdapterConfig:

parseMode?: "Markdown" | "MarkdownV2" | "HTML" | "none";

• Default remains "Markdown" — fully backward compatible with fix(telegram): set parse_mode for markdown messages #245.
• "MarkdownV2" / "HTML" are passed through to Telegram unchanged.
• "none" omits parse_mode, sending content as plain text (useful as a safety valve).

The markdown converter is unchanged; this PR only exposes a knob. Callers whose upstream renderer produces MarkdownV2/HTML/plain-text can now tell the adapter not to ask Telegram to parse legacy Markdown entities.

Tests

• New: parseMode: "MarkdownV2" and "HTML" round-trip into the request body.
• New: parseMode: "none" omits parse_mode entirely.
• New: invalid parseMode value throws ValidationError from the constructor.
• Existing parse_mode tests continue to pass with the unchanged default.

pnpm turbo build --filter @chat-adapter/telegram and pnpm vitest run (105 tests) both clean.

[vercel]

@Koshkoshinsk is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[Koshkoshinsk]

Closing — going to handle this on our caller side instead, no upstream change needed for now.

[Koshkoshinsk]

Reopening — apologies for the churn, the close was unintended on my end. Branch restored.

[vercel]

Additional Suggestion:

TelegramParseMode type is not re-exported from the package's public API, making it impossible for consumers to import.