feat(sqlalchemy-spanner): export MAX_SIZE constant

[sakthivelmanii]

Why?

Promoting _max_size to public MAX_SIZE (and dialect.max_size) allows cross-dialect applications to inspect Spanner's STRING(MAX) limit through the public API. It eliminates private symbol imports while preserving full backward compatibility for existing code.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request exposes the MAX_SIZE constant from the sqlalchemy_spanner module, making it publicly accessible and integrating it into the SpannerDialect class. It also adds a unit test to verify the export and the behavior of the int_from_size helper function. There are no review comments, so I have no feedback to provide.