bug: Incorrect serialization of numeric values

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* **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](https://github.com/magicstack/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'
```

Start in tests/test_codecs/test_numeric and add 10000 to the cases in test_numeric to reproduce the failure. Trace the numeric serialization logic responsible for returning 1E+4 instead of 10000, then run the test and confirm the existing cases still pass with the expected decimal string preserved.

Written by the indexing model from the issue text.