feat(telegram): expose parseMode config option

[Koshkoshinsk]

Re-opens #366 — that PR was closed in error and GitHub will not let me reopen it after the branch was deleted. Same commit, same diff.

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`:

```ts
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.

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Additional Suggestion:

TelegramParseMode type is not re-exported from the package entry point, making it inaccessible to consumers of @chat-adapter/telegram.

[Koshkoshinsk]

Good catch — pushed 048b951 adding TelegramParseMode to the type re-export block in src/index.ts. Build + 105 tests still clean.