Fix/join single presFix QueryBuilder JOIN state when using singleerve joins

[ljcjclljc]

Fix QueryBuilder JOIN state when using single

Summary

• Preserve JOIN clauses when converting a QueryBuilder to a single-result builder.
• Add regression tests for model and Row results with LEFT JOIN + single().
• Cover the PostgreSQL, MySQL, and SQLite test sections.

Problem

TransformBuilder::single() converts a non-single builder into a single-result builder. The conversion constructor copied the existing query state except for joins_.

As a result, a query such as:

QueryBuilder<Users>{}
    .from("users")
    .select("wallets.amount")
    .leftJoin("wallets", "users.user_id", "wallets.user_id")
    .limit(1)
    .single()
    .execSync(clientPtr);

lost its LEFT JOIN clause. The generated SQL still referenced wallets.amount, causing a database error or incorrect query behavior.

Changes

Copy joins_ in the TransformBuilder<T, SelectAll, true> conversion constructor:

this->joins_ = tb.joins_;

This keeps FROM, JOIN, filters, ordering, limit, and offset unchanged when .single() only changes the result cardinality.

Tests

• Added model-result regression tests for LEFT JOIN + single().
• Added Row-result regression tests that select wallets.amount.
• Added tests to the PostgreSQL, MySQL, and SQLite sections of orm_lib/tests/db_test.cc.
• SQLite database tests passed: 124 assertions in 3 test cases.
• clang-format and git diff --check passed.

Fixes #2577