`default_value` on dimension links doesn't work for non-string columns

[shangyian]

Background

A default_value on a dimension join link wraps the projected dimension column in COALESCE(column, default_value), so rows where a LEFT JOIN found no match render a fallback instead of NULL.

Problem

However, this setting only works when the dimension column is a string:

• The field is typed str at every layer, so default_value: 0 fails validation and you have to write "0".
• The literal is always wrapped in single quotes, so "0" against an integer column produces COALESCE(col, '0')

The above mean that the feature is unusable on any non-string dimension column -- you can't author the correct value, and the workaround generates invalid SQL.

Reproduction

Link a fact to a dimension whose attribute is an integer, with a numeric default:

dimension_links:
  - type: join
    dimension_node: my_dim
    join_type: left
    join_on: my_fact.dim_id = my_dim.dim_id
    default_value: 0          # rejected at validation — must be "0"

Then query a metric grouped by that integer attribute. With default_value: "0", the generated SQL contains:

COALESCE(t2.some_int_col, '0') AS some_int_col

which fails at execution.

Root cause

Typed as a string throughout:

• DimensionLink.default_value — database/dimensionlink.py:87 (Mapped[Optional[str]])
• API models — models/dimensionlink.py:73,88
• Deploy spec — models/deployment.py:226

And emission hardcodes string quoting in build_dimension_col_expr (construction/build_v3/measures.py:1049-1056):

if default_value is not None:
    coalesce_func = ast.Function(
        ast.Name("COALESCE"),
        args=[col_ref, ast.String(f"'{default_value}'")],
    )

The legacy v2 path has the same pattern (construction/build_v2.py:~1934).

Suggested fix

Make emission type-aware by building the literal from the target dimension column's type, or cast it, rather than assuming string. Widening the authored type is the second half, so default_value: 0 no longer needs quoting.