feat(reactions): support custom emoji reactions

[errogaht]

Problem

get_message_reactions already exposes custom reactions as
custom:<document_id>, but send_reaction always constructs
ReactionEmoji. Passing the value returned by the read API back into the write
API therefore cannot reproduce the custom reaction.

This breaks an otherwise useful read-modify-write flow for MCP clients: they can
identify a Telegram custom reaction, but cannot send the same reaction.

Change

• Accept custom:<positive document id> in send_reaction.
• Build a Telethon ReactionCustomEmoji for that representation.
• Set add_to_recent=True, matching the behavior expected when a custom
  reaction is selected from Telegram's reaction picker.
• If Telegram rejects that optional picker-state flag with
  REACTION_INVALID, retry the same custom reaction once without the flag.
  This handles private-chat behavior without hiding errors for normal Unicode
  reactions.
• Preserve the existing Unicode emoji path unchanged.
• Reject malformed custom identifiers before resolving a peer or sending a
  request.

The input representation intentionally matches the existing
get_message_reactions output, so custom reactions now round-trip without a
new tool field or breaking schema change.

Validation

• uv run pytest tests/test_reactions.py -q — 7 passed.
• uv run pytest — 170 passed.
• uv run black --check telegram_mcp/tools/messages.py tests/test_reactions.py
• uv run flake8 telegram_mcp/tools/messages.py tests/test_reactions.py --count --select=E9,F63,F7,F82 --show-source --statistics
• git diff --check

The regression coverage includes:

• unchanged Unicode reaction behavior;
• successful custom:<document_id> conversion;
• the narrow retry without add_to_recent;
• malformed and non-positive custom document IDs.