fix(postgres): strip psql backslash commands from pg_dump output

[endoze]

PostgreSQL 17.6+ emits psql-only directives like \restrict and \unrestrict in pg_dump output. These cause syntax errors when the schema file is loaded back via db.Exec() since they are not valid SQL. This filters them out during both dump and load so schema files remain portable and loadable without psql.

Closes #860

[fasmat]

Hi @endoze

Thanks for your contribution! Change LGTM.

[Copilot]

Pull request overview

Filters out psql backslash meta-commands (e.g. \restrict, \unrestrict) from pg_dump-produced schema SQL so schema dumps can be reloaded via db.Exec() without syntax errors (PostgreSQL 17.6+ compatibility).

Changes:

• Strip psql backslash directives during schema dump (genericDumpSchema) before writing to the output writer.
• Strip psql backslash directives during schema load (genericLoadSchema) before executing.
• Add unit tests for the stripping behavior and integration with genericDumpSchema.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dialect_common.go	Adds stripPsqlBackslashCommands and applies it in schema dump/load paths.
dialect_test.go	Adds tests validating backslash-command stripping behavior.

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