fix: precision loss when decoding binary NUMERIC values

[aminghadersohi]

Description

With the binary protocol, numeric_in_binary decodes NUMERIC values as Decimal(raw_value).scaleb(-scale). scaleb rounds to the current decimal context, which is 28 significant digits by default. The fix passes a 39-digit context to scaleb. That is enough for any 8- or 16-byte NUMERIC value, so every digit is kept and the exponent is still -scale.

Motivation and Context

A NUMERIC column with more than 28 significant digits is silently rounded on read:

stored (NUMERIC(38,18)):  12345678901234567890.123456789012345678
returned:                 12345678901234567890.12345679

This was reproduced against Redshift Serverless (1.0.434008). The value is stored exactly; psycopg2 reads it back exactly. Writes are unaffected because numeric_out sends the full text form. There is no existing issue for this.

Testing

• New test_numeric_in_binary_is_exact_beyond_28_digits in test/unit/datatype/test_data_in.py. It covers 38-digit, negative, smallest-scale, 39-digit (int128 max) and 8-byte values, and asserts exact equality and the exponent. The existing numeric tests use isclose(rel_tol=1e-6), which cannot detect this.
• Without the fix, 3 of the 5 new cases fail (the values with more than 28 digits). With the fix, all 77 tests in test_data_in.py pass.
• pytest test/unit: 2043 passed, 13 skipped. The 2 failures in test_browser_idc_auth_plugin.py also fail on the unmodified tree in that environment.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• Local run of ./build.sh succeeds (not run)
• Code changes have been run against the repository's pre-commit hooks (not run)
• Commit messages follow Conventional Commit Specification (the PR title does; the single commit message predates that and is not prefixed)
• I have read the README document
• I have added tests to cover my changes
• I have run all unit tests using pytest test/unit and they are passing (all pass except the 2 pre-existing test_browser_idc_auth_plugin.py failures noted above)

CHANGELOG.md is not edited: it is maintained at release time.

License

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