fix(client): use proper exception instead of assert for US endpoints

[lorenjphillips]

Problem

The binance.us staking endpoints use assert statements for region validation:

assert self.tld == "us", "Endpoint only available on binance.us"

This is problematic because assert statements are completely disabled when running Python with the -O (optimize) flag, which is common in production deployments. When this happens the validation silently disappears and users get confusing API errors from Binance instead of a clear client-side error.

Solution

Added BinanceRegionException - a proper exception class that:

• Can't be disabled by interpreter flags
• Gives callers a specific exception type to catch
• Includes structured info (required_tld, actual_tld, endpoint_name)

# Now users can handle this properly
try:
    client.get_staking_asset_us()
except BinanceRegionException as e:
    print(f"Wrong region: need {e.required_tld}, got {e.actual_tld}")

Changes

• Added BinanceRegionException to exceptions.py
• Added _require_tld() helper to BaseClient
• Replaced 6 asserts in client.py and 6 in async_client.py
• Added tests covering all affected endpoints

Testing

All 15 new tests pass. Ran linting with ruff, no issues.

tests/test_region_exception.py - 15 passed