feat: expose birthday in get_full_user

[akhromovRT]

What

Surfaces birthday from UserFull in the get_full_user tool output.

Why

Telethon's UserFull.birthday exposes the contact's date of birth when:

• the user has Telegram Premium,
• has set a birthday on their profile, and
• has allowed contacts to see it.

The year component is optional (often hidden by the user). Currently get_full_user does not expose this field, even though it sits right next to fields the tool already surfaces (bio, personal_channel, verified, premium, common_chats_count).

How

Adds 18 lines inside get_full_user (telegram_mcp/tools/profile.py):

• reads full_user.birthday via getattr (forward-compatible if Telethon ever omits the attribute),
• formats output as ISO 8601 YYYY-MM-DD when the year is present,
• falls back to vCard RFC 6350 --MM-DD for year-less dates,
• emits None when birthday is unavailable.

The new field is placed next to personal_channel in the result dict, keeping the order logical.

Testing

• PYTHONPATH=. uv run pytest -q --ignore=tests/test_media_album.py → 99/99 passed on this branch.
• tests/test_media_album.py shows 6 pre-existing failures on origin/main (verified by stashing the patch and re-running on a clean checkout) — they are infra-related (Error: 'account' is required) and unrelated to this change.
• Manual verification on a Premium contact with a set birthday: confirms a non-null birthday field in the tool output.

Notes

Follows the same minimal-patch style as #128 (fix: handle MessageReplyStoryHeader without reply_to_msg_id attribute).