AfterShip / clickhouse-sql-parser / 27258515509

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Fix nil-pointer dereference when formatting errors at EOF (#273)

Error messages built with `fmt.Errorf("... %q", p.last().String)` panic when
the lexer is at EOF, because `p.last()` is nil. Inputs like `ALTER `,
`SELECT*FROM A(0A` and `SET A=` triggered this.

Add a nil-safe `lastTokenString()` helper (mirroring `lastTokenKind()`) that
returns "<EOF>" when there is no current token, and use it in the 11 error
paths in parser_table.go and parser_alter.go. Success paths where last() is
provably non-nil are left untouched.

Add the inputs above to TestParser_InvalidSyntax to reproduce: they panicked
before the fix and now return a proper error.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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