Add integration tests for Decimal scanning in query and database/sql clients

[Copilot]

The issue requests support for scanning YDB Decimal values using types.Decimal as the destination type in both query client and database/sql. This functionality already exists but lacked test coverage.

Changes

• Added tests/integration/decimal_scan_test.go with comprehensive integration tests:
  • Query client: Direct row.Scan(&decimal) with various precision/scale combinations, negative values, NULL handling
  • Database/SQL: Both table service and query service modes with same coverage

Usage

// Query client
row, err := db.Query().QueryRow(ctx,   
    `SELECT Decimal('123456789.987654321', 33, 12)`,  
    query.WithIdempotent(),  
)  
var dst types.Decimal   
err = row.Scan(&dst)
// dst.Precision == 33, dst.Scale == 12

// Database/SQL
row := db.QueryRowContext(ctx, `SELECT Decimal('123456789.987654321', 33, 12)`)
var dst types.Decimal  
err = row.Scan(&dst)

No implementation changes needed - the underlying support in internal/value/value.go (line 615-619) and internal/table/scanner/scanner.go (line 900-901) already handles decimal.Decimal type casting correctly.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>feat: support YDB Decimal as destination type in query result and database/sql result</issue_title>
<issue_description>Example using query-client and Scan API

row, err := db.Query().QueryRow(ctx,   
    `SELECT Decimal('123456789.987654321', 33, 12)`,  
    query.WithIdempotent(),  
)  
if err != nil {  
    panic(err)  
}  
var dst types.Decimal   // or other type
err = row.Scan(&dst)  
// check value, precision, scale

Example using database/sql client

var db *sql.DB // init outside
row := db.QueryRowContext(ctx, `SELECT Decimal('123456789.987654321', 33, 12)`)
var dst types.Decimal  // or other type
err = row.Scan(&dst)  
if err != nil {  
    panic(err)  
}  
// check value, precision, scale
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes feat: support YDB Decimal as destination type in query result and database/sql result #1937

---

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[github-actions]

summary

Inferred base version: v3.118.2
Suggested version: v3.118.3

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.23%. Comparing base (70fd60b) to head (5eb7e83).

❗ There is a different number of reports uploaded between BASE (70fd60b) and HEAD (5eb7e83). Click for more details.

HEAD has 503 uploads less than BASE

Flag	BASE (70fd60b)	HEAD (5eb7e83)
macOS	24	2
unit	71	6
go-1.25.x	79	3
go-1.21.x	69	3
windows	23	2
ubuntu	101	2
experiment	10	0
ydb-nightly	10	0
ydb-latest	21	0
integration	67	0
ydb-25.2	24	0
ydb-24.4	22	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1938       +/-   ##
===========================================
- Coverage   74.01%   47.23%   -26.79%     
===========================================
  Files         392      391        -1     
  Lines       34343    34208      -135     
===========================================
- Hits        25418    16157     -9261     
- Misses       7807    17453     +9646     
+ Partials     1118      598      -520     

Flag	Coverage Δ
experiment	?
go-1.21.x	47.19% <ø> (-25.54%)	⬇️
go-1.25.x	45.07% <ø> (-28.92%)	⬇️
integration	?
macOS	47.20% <ø> (-0.03%)	⬇️
ubuntu	47.19% <ø> (-26.82%)	⬇️
unit	47.23% <ø> (-0.02%)	⬇️
windows	47.22% <ø> (-0.01%)	⬇️
ydb-24.4	?
ydb-25.2	?
ydb-latest	?
ydb-nightly	?

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