fix(sqlalchemy-spanner): fix get_multi_indexes crash on SEARCH indexes

[sakthivelmanii]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request updates the Spanner dialect to exclude SEARCH indexes from the results of get_multi_indexes and improves robustness when handling null or missing column orderings. It also adds unit tests to verify these changes. The reviewer suggested a cleaner approach for handling columns without explicit sorting by omitting them from the column_sorting dictionary entirely instead of mapping them to None, which aligns better with SQLAlchemy standards. Corresponding updates to the unit tests were also recommended.