[rocksnow1942]

_designate_type() raised KeyError for column types it doesn't map. Since get_multi_columns() reads all the tables of a schema in one query, one such column breaks the reflection of the whole schema. These column types fail on the emulator (1.5.58):

Column	SPANNER_TYPE	Error
vector	ARRAY<FLOAT32>(vector_length=>3)	KeyError: 'FLOAT32>(vector_length=>3'
proto	`my.package.Message`	KeyError: '`my.package.Message`'
enum	`my.package.Status`	KeyError: '`my.package.Status`'

(The issue also mentions INTERVAL and STRUCT, but neither can be a column type.)

This PR:

• reads the element type of an array up to the first >, so vector columns are reflected as ARRAY(REAL) or ARRAY(Float);
• reflects types the dialect doesn't recognize as NullType with an SAWarning, as SQLAlchemy's built-in dialects do. Arrays of such types are reflected as ARRAY(NullType()).

The NullType fallback was first proposed by @waiho-gumloop in #16622, which was moved to draft after #17911 fixed its TOKENLIST part. This PR supersedes it and adds the vector, proto and enum cases.

Tested with the unit and mockserver tests and the compliance suite against the emulator.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #18839

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[gemini-code-assist]

Code Review

This pull request enhances the Spanner dialect's type designation to support vector columns and gracefully handle unrecognized types like PROTO and ENUM by reflecting them as NullType with a warning. Unit and integration tests have been added to verify these changes. Feedback on the changes highlights a potential robustness issue where using str_repr.index('>') could raise a ValueError and crash schema reflection if the character is missing; using find with a fallback is recommended.