fix(cli-go): avoid requiring local stack for gen types with explicit --db-url

[mikkelam]

fix(cli-go): avoid requiring local stack for gen types with explicit --db-url

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

supabase gen types --db-url ... can fail with supabase start is not running when the provided DB URL points to localhost local stack defaults, even though --db-url is explicitly set and the target DB is reachable without supabase start.

Issue: #5283

What is the new behavior?

When --db-url is explicitly provided, gen types no longer enforces local stack running checks.
It uses the provided connection directly, including localhost targets managed outside supabase start (for example Docker Compose).

Added regression coverage in internal/gen/types/types_test.go for explicit --db-url on localhost.

Additional context

Scope is intentionally narrow:

• behavior change applies only to explicit --db-url in gen types
• default local/linked flows are unchanged