[Mysql-kit] Fix introspection dropping a zero default on numeric columns

[Otto-Deviant1904]

Fixes #5911.

introspect-mysql.ts's column() function decides whether to append .default(...) to a generated column definition using a plain truthy check on the parsed default value: defaultValue ? '.default(...)' : ''.

That breaks for a legitimate default of 0 (numeric types) or false (boolean), since both are falsy in JS. int and tinyint already guard against this correctly with typeof defaultValue !== 'undefined', but smallint, mediumint, bigint, boolean, double, float, and real still used the truthy check, so a NOT NULL DEFAULT 0 column on any of those types silently loses its .default() during introspection — matching the report exactly (a MySQL FLOAT NOT NULL DEFAULT 1.5 column round-trips fine, but the same column with DEFAULT 0 doesn't).

Fixed by applying the same typeof defaultValue !== 'undefined' check int/tinyint already use, to the remaining affected types.

Verified against a real mysql:8.0 container:

• Before the fix: float().notNull().default(0) introspects as float().notNull() — default silently dropped.
• After the fix: introspects correctly as float().default(0).notNull().

Added two regression tests to tests/introspect/mysql.test.ts: one matching the reported float case, one covering the other affected types (smallint, mediumint, bigint, double, real) with a zero default. Ran the full introspect/mysql.test.ts suite — all 12 pass.

Left boolean out of the regression test even though I fixed that code path too — MySQL reports BOOLEAN columns as tinyint(1) in information_schema, not boolean, so that branch turned out to be unreachable via real introspection and hits an unrelated, pre-existing bug in the generated file's import list when exercised directly. Didn't want to fix an unrelated issue in this PR, but happy to file it separately if useful.