Fold PostgresqlMigrator.AssertValidIdentifier onto the shared helper (weasel#416)

[jeremydmiller]

Follow-on to #420, which left PostgreSQL on its own hand-rolled loop — the reasoning there was that folding it in would be a no-behaviour-change edit to code shipped in 9.23.0. That turns out to be half right, so the delta is worth stating plainly up front.

Unchanged: the rules PostgreSQL already had come back identical — null/empty/whitespace-only, whitespace anywhere, ", ; — with the same reason strings, so PostgresqlIdentifierInvalidException.Reason and the exception message are byte-for-byte what they were. The length check and PostgresqlIdentifierTooLongException stay in the provider, since NameDataLength is PostgreSQL's own and configurable.

Changed: the shared helper also rejects ', and PostgreSQL did not.

That is not uniformity for its own sake. Table.cs:584 — DefaultValueFromSequence — writes a sequence's DbObjectName into a string literal:

return DefaultValueByExpression($"nextval('{sequenceName}')");

so a sequence name carrying a ' closes that literal and the rest of the DEFAULT clause is whatever the name says next. PostgreSQL's introspection queries do parameterise names (Table.FetchExisting, Function), which is why the exposure here is narrower than on SQL Server, Oracle and SQLite, where the existence checks interpolate them — but it is not zero.

An apostrophe in a schema object name has no legitimate use that survives Weasel's quoting anyway: SchemaUtils.QuoteName only quotes keywords and names containing an uppercase character, so such a name is written out raw. Still, it is a tightening layered on a released one, and anyone with a ' in an object name will now get an exception where 9.23.0 gave them DDL.

The three single-quote cases are added to assert_identifier_rejects_quote_and_semicolon rather than to a new test, since it is the same rule.

Regression pass

Full suites on net9.0, PostgreSQL container from docker compose up:

Suite	Result
Weasel.Postgresql	849 passed, 3 pre-existing skips (846 before, plus the three new cases)
Weasel.Core	75 passed
Weasel.EntityFrameworkCore	106 passed, 1 skip
Weasel.CommandLine	23 passed

The other four providers are untouched by this — their behaviour landed in #420.

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