fix(postgresql): reject quote and semicolon in AssertValidIdentifier (weasel#416)

[jeremydmiller]

Closes #416.

Finishes what 26421ba (9.21.1) left on the issue. That change fixed the partition bound literals; this one closes the identifier side and clears the remaining checkboxes.

The gap

AssertValidIdentifier is the only identifier check in the stack — DbObjectName and PostgresqlObjectName contain no validation at all — yet it rejected only null/whitespace, a literal space, and over-length. It permitted the two characters that let a name escape the statement it is written into:

• " closes a quoted identifier. Weasel quotes identifiers without doubling an embedded quote, so a name carrying one does not stay inside its own quotes.
• ; ends the statement and starts another.

Whitespace is now rejected in full rather than just the literal space character — the old check missed tab, newline and carriage return, so a newline could introduce a -- comment into an otherwise unquoted name.

Also

• PostgresqlIdentifierInvalidException gained a constructor that names the rule that was broken, so the message says which character was the problem instead of just "not valid". The existing single-argument constructor is untouched. The message also stopped blaming Marten for a decision that is Weasel's.
• Doc comments on DbObjectName and PostgresqlObjectName now say plainly that they do not validate. PostgresqlObjectName.From is sometimes assumed to be a sanitizing boundary; it quotes for rendering, which is not the same thing.

Verification

This is the regression pass 26421ba asked for before tightening this method.

• Full Weasel.Postgresql suite green: 846 passed, 3 pre-existing skips.
• Weasel.Core green (50).
• No name Weasel or its consumers generate was affected. assert_identifier_still_accepts_ordinary_names pins mt_* names, $, leading underscores, mixed case, non-ASCII letters, dashes and dotted partition suffixes as still valid.

Deliberately not ridden along

The other providers' AssertValidIdentifier, which #416 listed as unestablished. Now established: SqlServerMigrator's is an empty method body (// Nothing yet), Oracle and MySql check length only, Sqlite checks null/whitespace and length. None reject " or ;. Bringing four providers' validation into line is a behaviour change per provider and wants its own change.

SchemaUtils.QuoteName and the other quote-without-escape helpers from the issue's table. QuoteName cannot simply double an embedded " — it only quotes at all when the name is a keyword or has an uppercase character, so the fix also has to force quoting for such names, and PostgresqlProvider.ToQualifiedName's pre-quoted-passthrough guard is evidence that quote-wrapped names do circulate on that path. Object names now can't carry a quote past AssertValidIdentifier; what remains exposed are schema names, index columns and collations, which don't route through it.

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