bug: Incorrect serialization of numeric values

• asyncpg version: 0.29.0
• PostgreSQL version: 13
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: no SaaS, yes reproducable with local postgres
• Python version: 3.10
• Platform: linux
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: none
• Can the issue be reproduced under both asyncio and
  uvloop?: yes

This happens when query selects a numeric column. Numbers like 10000, 100000, etc. get serialized in scientific notation. For example 10000 becomes 1E+4.

To reproduce the error, you can do the following:

Inside tests/test_codecs/test_numeric if 10000 is added in cases then the test fails, due to incorrect serialization logic.

    async def test_numeric(self):
        # Test that we handle dscale correctly.
        cases = [
            '0.001',
            '0.001000',
            '1',
            '1.00000',
            '10000', # new test case
        ]

fail error:

line 614, in test_numeric
    self.assertEqual(str(res), case)
AssertionError: '1E+4' != '10000'

tests/test_codecs/test_numeric から始め、失敗を再現するために test_numeric のケースに 10000 を追加します。10000 ではなく 1E+4 を返す原因となっている数値シリアライゼーションのロジックを追跡し、その後テストを実行して、既存のケースが期待される小数文字列を維持したまま引き続き通過することを確認します。