Unescape doubled single quotes in scaffolded string default values

[ilkertskn]

SqlServerDatabaseModelFactory and SqliteDatabaseModelFactory strip the surrounding quotes from a string literal default (e.g. ('It''s')) but leave the SQL escape sequence '' in place, so the scaffolded model gets HasDefaultValue("It''s"). The migrations SQL generator then escapes the value again and emits DEFAULT N'It''''s', changing the default to It''s.

Replace '' with ' after removing the enclosing quotes in both factories, and add regression tests covering plain, N-prefixed, CONVERT-wrapped and single-quote-only literals.

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