Use " AS " when aliasing tables

[tempoz]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Addresses #6391

Uses " AS " instead of " " between a table and its alias.

User Case Description

As described in the related issue, #6391:

My specific use case is that I want to be able to use the table alias in clause.OnConflict.DoUpdates because the table names I'm working with are mixed case, which means I need to manually quote them to reference them directly in the expressions for those updates when I'm using postgres (and I have to use a table name or alias to disambiguate the column from the row that failed to insert, which is available as excluded), which I do not want to do.

[propel-code-bot]

This ensures stmt.QuoteTo renders table aliases with an explicit AS, keeping alias handling predictable across SQL dialects when dealing with quoted or mixed-case identifiers.

Affected Areas

• statement.go
• clause/expression_test.go

This summary was automatically generated by @propel-code-bot

[tempoz]

The failures in the checks seem ignorable:

• the linter is complaining about not checking the error returned by writer.WriteString, but that matches both the behavior that was there before when we were calling writer.WriteByte as well as all other calls to writer.WriteString in the rest of the function
• the mysql test failures seem wholly unrelated to this change.