Cross-backend edge-case consistency test matrix

[yancya]

Background

The three backends were written years apart and their edge-case behavior drifted (see the sibling correctness issues: PG empty tuples, PG/MySQL/BQ identifier escaping, MySQL Infinity). Nothing today pins down "what all backends must agree on" — each test file only tests its own backend's status quo.

Prerequisites

• The individual correctness fixes should land first (PG empty tuples, PG identifier quoting, MySQL/BQ backtick escaping, MySQL Infinity) — this issue locks in their outcomes. Check which are closed before starting.

Task

Add one cross-backend test file (e.g. test/consistency_test.rb, test-unit like the rest) that runs a shared matrix of edge cases against all three backends and asserts either the common behavior or the documented, intentional divergence:

Case	Expectation
nil value in a tuple	SQL NULL in output, all backends
column with only nils, no explicit type	ReasonlessTypeError (or documented alternative), all backends
empty tuples + explicit types	valid 0-row relation, all backends
empty tuples, no types	TypeNotFoundError, all backends
mixed types in a column	ReasonlessTypeError, all backends
single column	valid output (BQ adds dummy column)
quote character in column name	correctly escaped per backend
quote character in string value	correctly escaped per backend
Float::INFINITY / NAN	per-backend documented behavior (PG literal, BQ handling, MySQL error)
BigDecimal round-trip precision	value survives to the literal without float truncation

Acceptance criteria

• New test file covering the matrix above for BigQuery, Postgresql, and MySQL
• Any intentional divergence is asserted AND written down in a README "Edge cases" / "Errors" subsection (extend the existing Errors section)
• bundle exec rake runs the new file via the existing default task glob (test/*_test.rb)
• CI green on Ruby 3.2/3.3/3.4/4.0

Verification

• bundle exec rake green locally and in CI
• The matrix table in this issue is fully mirrored by test cases (reviewer can diff table rows against test data entries)