format_decimal: reject unsupported numbering systems with group_separator=False

[glaziermag]

With group_separator=False, format_decimal silently accepts an unsupported numbering system when the formatted fractional part is zero. Grouped output and output with a nonzero fractional part already raise UnsupportedNumberingSystemError, because those rendering branches fetch number symbols and validate the system.

Validate the numbering system immediately after parsing the locale, before formatting. The regression test covers zero, an integer, and a value rounded to an integer, with grouping enabled and disabled.

Minimal reproduction on unmodified master at e2a17919f0eb988bfea2883b00793ca4d5d37c4f:

from babel.numbers import format_decimal

format_decimal(1, locale="en_US", group_separator=False, numbering_system="unknown")

Actual: '1'. Expected: UnsupportedNumberingSystemError, as documented in the project's format_decimal API contract.

Validation on Python 3.14.4 with the project's CLDR 48.2 data:

• Regression test before the fix: 3 failed, 3 passed; after the fix: all 6 passed.
• Number tests: 137 passed.
• Full suite, python -X utf8 -m pytest -q: 7,837 passed, 7 skipped, 2 xfailed.
• git diff --check: passed.

Prepared and tested with OpenAI Codex.