Disable TON network in Changelly and SideShift swap allowlists

[andycreed0x]

Purpose

Remove TON from the curated swap allowlists in both Changelly and SideShift integrations. New TON swap orders are rejected at the tool/CLI/MCP layer; existing TON swaps already on disk remain pollable via `*_status` until they reach a terminal state.

Description

Soft-disable approach: TON is removed from the source-of-truth data structures (`EXTERNAL_USDT_IDS`, `NETWORK_TO_USDT_ID`, `_ADDRESS_PATTERNS`, `ALLOWED_PAIRS`). The existing `CHANGELLY_ALLOW_ALL_PAIRS=1` / `SIDESHIFT_ALLOW_ALL_NETWORKS=1` env-var overrides remain the escape hatch for debugging or power users.

Main Changes

• ♻️ Remove `"usdton"` from `EXTERNAL_USDT_IDS`, `NETWORK_TO_USDT_ID`, and `_ADDRESS_PATTERNS` in `changelly.py`
• ♻️ Remove `("usdt", "ton")` from `ALLOWED_PAIRS` in `sideshift.py`
• ♻️ Remove `"ton"` from `click.Choice` in `cli/changelly.py` and docstrings in `cli/sideshift.py`
• ♻️ Remove `"ton"` from the three `external_network` MCP enums in `server.py` (changelly_quote / changelly_send / changelly_receive)
• 📝 Update all prose references ("6 chains" → "5", "7 chains" → "6", TON removed from memo-network examples) across `server.py`, `tools.py`, `cli/`
• ✅ `tests/test_changelly.py`: `ALLOWED_PAIRS` size 12 → 10, drop TON parametrize rows, add `"usdton"` to the filter-drain assertion
• ✅ `tests/test_sideshift.py`: drop `("usdt","ton")` from allowlist parametrize; add `assert "ton" not in` for both filtered networks and tokenDetails (raw fixture keeps TON to validate the filter)
• 📝 Fix two stale numeric comments missed in first pass (`changelly.py` docstring "6 variants" → "5", `test_sideshift.py` class docstring "7 chains" → "6")

⚠️ Breaking Changes

• Calling `changelly_send`, `changelly_receive`, or `changelly_quote` with `external_network="ton"` now raises `ValueError` / CLI `UsageError`.
• `network_to_asset_id("ton")` raises `ValueError`.
• SideShift send/receive with `deposit_network="ton"` or `settle_network="ton"` raises `ValueError`.
• In-flight TON swaps already on disk are not affected — `changelly_status` / `sideshift_status` do not re-validate the allowlist.

Checklist

• Added/updated tests (if necessary)
• Added/updated relevant documentation (if necessary)