Add UpdateMultiple for bulk updates

[henkmollema]

Summary

Adds UpdateMultiple<T> / UpdateMultipleAsync<T> extension methods for bulk updates, modeled on EF Core''s ExecuteUpdate but kept simple and consistent with the rest of the Dommel API. They issue a single UPDATE ... SET ... WHERE statement without loading entities first.

await connection.UpdateMultipleAsync<Product>(
    update => update.SetProperty(p => p.Name, "<removed>")
                    .SetProperty(p => p.AmountInStock, p => p.AmountInStock - 1),
    x => x.AmountInStock == 0);

The properties to assign are described with a fluent SetProperty chain — either a constant value or a value selector that may reference entity columns and use arithmetic. Rows are filtered with a LINQ predicate. The methods return the number of affected rows (matching DeleteMultiple and ExecuteUpdate), and both sync and async variants are provided.

Implementation notes

• New SetPropertyCalls<TEntity> fluent builder with both SetProperty overloads (constant value + value selector).
• Single SqlExpression instance (obtained via the SqlExpressionFactory) builds both the set-clause and where-clause, so their auto-numbered parameters share numbering and never collide — and the where-clause stays JSON-aware under Dommel.Json.
• SqlExpression now translates arithmetic operators (+ - * / %), enabling value selectors like p => p.Stock - 1 (also usable in Where).
• Assignment targets are emitted unqualified (set [FullName] = ...) while where-clause columns stay qualified — some databases (e.g. Postgres) reject a table-qualified assignment target.
• Constant values vs. rendered-SQL fragments are distinguished by whether the value expression references the entity parameter, so a constant string value is correctly parameterized.

Known limitations (documented in XML comments)

• Value selectors support column op constant arithmetic; column op column is not supported (consistent with existing Where behavior).
• Nested/precedence-sensitive arithmetic is not parenthesized.

Testing

• Dommel.Tests: 206 passed (incl. new UpdateTests SQL-generation cases across SQL Server / MySQL / Postgres and an arithmetic-in-Where regression test); no regressions from the arithmetic-routing change.
• Dommel.Json.Tests: 35 passed.
• New sync + async integration tests added under the existing [ClassData(typeof(DatabaseTestData))] pattern (require local SQL Server / MySQL / PostgreSQL; not run in this environment).

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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