fix(groups): correct get_participants pagination

[antonskiter]

Fixes #170.

get_participants pagination has two independent defects; this fixes both in telegram_mcp/tools/groups.py.

1. iter_participants has no offset parameter. Telethon's signature is (entity, limit=None, *, search, filter, aggressive) — no offset, in the v1 source or the 1.44.0 docs, and the project requires telethon>=1.44.0. Since offset is passed unconditionally, every call raises TypeError.

2. limit does not bound basic groups. In telethon/requestiter.py:

if await self._init(**self.kwargs):
    self.left = len(self.buffer)

For _EntityType.CHAT, _ParticipantsIter._init buffers the whole participant list and returns True, so left is overwritten with the buffer length and the requested limit is discarded. Channels are unaffected — there _init returns falsy. So limit alone cannot bound a page.

Fix. Skip to the start of the requested page and stop once it is full, relying on neither offset nor limit. limit is still passed so channels do not over-fetch.

Verification. Against a live account on telethon 1.44.0:

• basic group, 6 members, page_size=2 — pages 1/2/3 each return 2 non-overlapping participants in list order, page 4 returns empty
• supergroup, page_size=3 — pages 1 and 2 each return 3, disjoint

Before the fix the same calls returned a formatted TypeError; with only defect 1 fixed, page=1, page_size=1 on the basic group returned all 6 participants.

No behaviour change for page=1, page_size=200 (the defaults) beyond it no longer erroring.