feat(isthmus): make unquoted identifier casing configurable in ConverterProvider

[nielspardon]

Summary

Adds constructor-based configuration of unquoted SQL identifier casing to ConverterProvider, so that isthmus consumers can control how unquoted identifiers are cased during parsing. The default remains Casing.TO_UPPER (no behaviour change).

Previously the only way to change this was to subclass ConverterProvider and override getSqlParserConfig() — as IsthmusEntryPoint already did with an anonymous class. That workaround is now replaced by a first-class constructor parameter.

Changes

ConverterProvider

• unquotedCasing is a new final field, consistent with executionBehavior
• getUnquotedCasing() — getter
• getSqlParserConfig() reads unquotedCasing instead of hard-coding Casing.TO_UPPER
• New constructors: ConverterProvider(Casing) and ConverterProvider(extensions, typeFactory, Casing) for the common cases; the existing 7-arg all-components constructor gains Casing as an 8th parameter. All narrower constructors default to Casing.TO_UPPER.

Propagation through the pipeline

The casing setting is applied consistently across both CREATE TABLE parsing and query parsing, so that the table name stored in a NamedScan matches the configured casing end-to-end.

Class	Change
SubstraitSqlToCalcite	New convertQueries(sql, catalog, ConverterProvider, operatorTable) overload; passes getSqlParserConfig() down to the statement parser
SqlToSubstrait	convert(sql, catalog) now uses the ConverterProvider overload of convertQueries
SubstraitCreateStatementParser	New processCreateStatements(ConverterProvider, sql) and processCreateStatementsToCatalog(ConverterProvider, ...) overloads; SqlParser.Config stays an internal detail
SqlExpressionToSubstrait	Uses processCreateStatements(converterProvider, tableDef)
IsthmusEntryPoint	Uses new ConverterProvider(unquotedCasing); anonymous ConverterProvider subclass removed

Test

UnquotedCasingTest verifies:

• The default casing is TO_UPPER and is reflected in getSqlParserConfig()
• new ConverterProvider(Casing) sets the casing correctly for all three Casing values
• End-to-end: with TO_UPPER a plan built from CREATE TABLE employees … / SELECT … FROM employees produces a NamedScan with name EMPLOYEES; with UNCHANGED it produces employees

Notes

SubstraitSqlStatementParser keeps SqlParser.Config as its parameter type — it is a low-level parse primitive used by multiple callers with different configs. ConverterProvider awareness belongs one level up, which is where it already sits.