feat(orm): select_sub / add_select subquery-as-column + table() setter

[tmgbedu]

Summary

Follow-up B from splitting #168. Adds subquery-as-column support to the async query builder:

posts = await Post.query().select("id", "title").select_sub(
    Category.query().select("name").where_column("categories.id", "posts.category_id").limit(1),
    "category_name",
).get()

• select_sub(builder, alias) and add_select(alias, subquery) render a subquery as an aliased column — (subquery) AS alias — with bindings emitted in SELECT position (before WHERE). add_select also accepts a callable that builds the subquery on a fresh builder.
• table(name) sets the target table on a bare builder. This is the setter the *Through / BelongsToMany with-count query paths already reference (previously dangling), so it resolves those calls and backs the callable form of add_select.

Existing string-based select() is unchanged.

Tests

• tests/masoniteorm/query/grammars/test_select_sub_grammar.py — grammar-level to_sql()/to_qmark() + binding-order assertions across all 4 dialects (incl. MSSQL TOP n); covers builder-instance and callable forms and the non-builder TypeError.
• tests/masoniteorm/sqlite/builder/test_sqlite_select_sub.py — real sqlite-backed correlated-column reads plus the table() setter.

Gates

• uv run pytest --ignore=tests/masoniteorm/postgres --cov → 1808 passed / 7 skipped, coverage 78.88% (≥ fail_under 68).
• ruff check + ruff format --check clean.
• Zero regression to existing string-based select().

Relationship to other PRs

Independent of #168 and #169 (correlated subqueries here use the 2-arg where_column already on main). Recommended overall order: #168 → #169 → this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[tmgbedu]

✅ Code Review Verdict: APPROVE (posted as comment — GH blocks self-approval)

Reviewed for correctness, binding order, alias emission, and regression. All gates reproduced locally on the PR branch.

Verified

• SELECT-position binding order is correct. BaseGrammar._compile_select evaluates process_columns(...) before process_wheres(...) (kwargs eval left-to-right), so subquery bindings precede WHERE bindings. get_bindings() == [1, "active"] is genuinely exercised and correct. No accumulation across re-compiles: each to_qmark() builds a fresh grammar and the subquery's to_qmark() reassigns (not +=) its _bindings.
• Alias emission (subquery) AS alias correct across all 4 dialects incl. MSSQL TOP n.
• add_select callable form constructs QueryBuilder(connection, grammar, processor) matching the real __init__ signature; non-builder input raises TypeError as tested.
• table() setter is genuinely new (no collision with any mixin/method on QueryBuilder) and correctly backs the previously-dangling chained .table() calls in HasManyThrough/HasOneThrough/BelongsToMany with-count paths. Full relationship suite passes.
• Reuses existing SubGroupExpression rather than inventing a new mechanism — good altitude.
• Zero regression / test quality: real sqlite-backed correlated reads + grammar-level assertions, no hollow mocks.

Gates reproduced (PR worktree)

• uv run pytest --ignore=tests/masoniteorm/postgres → 1808 passed / 7 skipped (matches report)
• New tests: 9 passed / 20 subtests
• ruff check + ruff format --check → clean

Non-blocking notes

• Alias in (subquery) AS {alias} is emitted unquoted — consistent with existing correlated where_column fragments; fine for normal identifiers. Not a blocker.
• With-count paths now activated rely on the general suite for coverage; dedicated tests tracked in backlog #886.

No correctness bugs found. Recommend merge (per split order #168 → #169 → this).