sql: implement binary collation check and expression coercibility res…

[elianddb]

…olution
Fix dolthub/dolt#11380

[itoqa]

Commit: 722da8b: 19 test cases ran, 19 passed ✅.

Summary

Coverage spans core text-function behavior, including left and right padding, Unicode and binary character handling, collation and precedence rules, NULL and invalid-input behavior, truncation, argument ordering, and concurrent session isolation. It includes normal flows, boundary and error cases, regression checks, and adversarial review of an expected coercibility concern.

Safe to merge — all exercised behaviors passed, and the only reported concern was determined to conflict with the intended collation rules rather than indicate a product defect. No PR-attributable regressions, new failures, or previously flagged failures remain.

Tests run by Ito

Result	Severity	Type	Description
✅	—	General	LPAD and RPAD returned the requested number of characters without splitting accented letters or emoji. Both functions also kept their correct left and right padding behavior.
✅	—	General	LPAD and RPAD correctly repeat a multibyte pad and use a complete character for the partial remainder. The results have the requested length and keep padding on the correct side.
✅	—	General	The SQL engine returned the expected collation and coercibility for same-collation, unspecified, binary, and conflicting pairs. It also chose the lower-coercibility operand when the two values differed by one level.
✅	—	General	Nested text and padding operations returned the expected collation and character-length results. The browser error was expected because the local target speaks MySQL, not HTTP.
✅	—	General	Malformed padding input returned an error without showing partial output. The next valid padding calls returned the correct values.
✅	—	General	Padding keeps the left and right directions separate when called through the SQL registry. Truncation and NULL values also produce the expected results.
✅	—	General	LPAD and RPAD keep the source, length, and pad arguments in the right roles after scalar-subquery rewrites. Direct and rewritten SQL returned the same left/right padding, NULL values, and invalid-length error.
✅	—	General	Two independent SQL sessions ran conflicting collation queries 100 times each. Both sessions returned the expected value and metadata every time, with no results leaking between sessions.
✅	—	Collation	LPAD and RPAD returned the source string collation and coercibility values expected by the SQL engine.
✅	—	Functions	CONCAT, CONCAT_WS, ELT, FIELD, REPLACE, LPAD, and RPAD returned their expected results in focused unit tests and the SQL engine integration suite.
✅	—	Functions	Padding returns NULL when the source, length, or pad value is NULL, and it reports an error when the length cannot be converted to a number.
✅	—	Functions	Concatenation, selection, comparison, and replacement with mixed text values returned the expected results and collation metadata. Equivalent nested expressions matched the direct expressions, and the focused Go tests passed.
✅	—	Padding	LPAD and RPAD added multibyte padding on the correct side and truncated the source at complete characters. The local SQL checks and the focused regression tests passed.
✅	—	Padding	LPAD and RPAD return empty strings for a zero length and SQL NULL for negative or NULL lengths. The focused regression tests also pass for both functions.
✅	—	Resolution	The resolver returned the expected collation and coercibility for lower-precedence values, unspecified operands, explicit conflicts, binary collations, and incompatible character sets.
✅	—	Rev	Binary padding returned the expected byte sequences. Truncating C3A9 to one byte returned C3, and padding 41 with C3A9 returned 41C3A9C3 without a malformed-string error.
✅	—	Rev	A latin1 column kept its collation through both padding functions. The left-padded value was xxxé, the right-padded value was éxxx, and the padded value had four characters.
✅	—	Rev	Verified acceptable by independent adversarial review: the reported expectation does not match what the code actually promises. Review notes: The finding's decisive premise, that a CONCAT of two differently and explicitly collated operands must itself have coercibility 0, conflicts with the repository's explicit aggregation contract and test: a conflict between equal-strength explicit collations yields CoercibilityNone (1). For the stated expression, the static call path is CollatedExpression (0 for each operand) through CONCAT's reduce…
✅	—	Rev	NULL inputs return NULL, and an empty pad returns an empty string when more characters are requested.

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