fix: support queries starting with parentheses

[olavloite]

Queries enclosed in parentheses (such as (SELECT 1)) had their command tag evaluated as an empty string by SimpleParser.parseCommand(sql). This caused PGAdapter to treat the query as empty and send an EmptyQueryResponse to the client, returning zero rows.

This change:

• Updates SimpleParser.parseCommand to skip leading opening parentheses before checking for WITH clauses and before reading the command keyword.
• Updates SimpleParser.isCommand to recognize parenthesized statements.
• Handles [NOT] MATERIALIZED clauses when skipping CTE definitions.
• Adds a defensive fallback in IntermediateStatement to use "SELECT" if SimpleParser yields an empty command tag for a statement Spanner classified as a query.

[gemini-code-assist]

Code Review

This pull request enhances SQL parsing in PGAdapter by supporting queries wrapped in parentheses, handling 'materialized' and 'not materialized' CTE hints, and falling back to 'SELECT' when Spanner identifies a statement as a query but the simple parser cannot determine the command tag. The review feedback suggests simplifying the keyword-eating logic for CTE materialization hints and changing the visibility of skipOpeningParentheses() to private for better encapsulation.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request enhances the SQL parsing capabilities of PGAdapter to correctly handle queries starting with parentheses (e.g., (SELECT 1)) and CTEs using recursive, materialized, or not materialized keywords. It updates SimpleParser to skip opening parentheses when identifying command tags and improves IntermediateStatement to fall back to a SELECT command tag if the parser cannot determine it but Spanner's parser identifies the statement as a query. Comprehensive unit and mock server tests have been added to verify these changes. No review comments were provided, so there is no additional feedback.