feat: include PostgreSQL error code in exceptions

[olavloite]

PostgreSQL error codes are now included in SpannerException when available.

The PostgreSQL error code is only filled with a non-null value if the database is a Spanner PostgreSQL-dialect database, and the error is one that has a corresponding error code in PostgreSQL.

See https://www.postgresql.org/docs/current/errcodes-appendix.html for a full list of all PostgreSQL error codes.

[olavloite]

cc @hengfengli

[gemini-code-assist]

Summary of Changes

Hello @olavloite, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the error reporting capabilities of the Spanner client library. By embedding PostgreSQL SQLSTATE error codes directly into SpannerException objects, it provides developers with more precise and actionable diagnostic information, especially when working with Spanner instances configured for PostgreSQL dialect. This change aims to streamline the debugging process and improve the overall developer experience by offering clearer insights into the root cause of database errors.

Highlights

• PostgreSQL Error Codes: SpannerException now includes PostgreSQL error codes when available, specifically for Spanner PostgreSQL-dialect databases, providing more granular error information.
• New API for Error Codes: A new method, getPostgreSQLErrorCode(), has been added to SpannerException to retrieve the SQLSTATE error code.
• Exception Handling Refactor: Many internal calls to SpannerExceptionFactory.newSpannerException have been replaced with SpannerExceptionFactory.asSpannerException for consistency and improved error propagation.
• Statement Context in Errors: The SpannerException class can now store the problematic SQL statement, which is included in the exception message for enhanced debugging.
• Test Coverage: An integration test (ITQueryTest.java) was updated to verify the correct retrieval of PostgreSQL error codes from SpannerException.

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[gemini-code-assist]

Code Review

This pull request adds support for including PostgreSQL error codes in SpannerException and refactors exception creation to use a new asSpannerException method. The changes are generally good, but there's a systematic issue with how asSpannerException is called. In many places, it's called with an argument that might already be a SpannerException. Based on existing code patterns in SpannerApiFutures, it appears asSpannerException is not idempotent, which means these calls will re-wrap exceptions. This is undesirable as it can obscure the original stack trace. I've added several comments to address this by adding checks to handle existing SpannerException instances directly.