fix: return UUID as string type for MySQL clients

[robll-v1]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #21512

What this PR does / why we need it:

The server was returning UUID values as strings, but the MySQL column type was still MYSQL_TYPE_UUID. Go MySQL drivers (e.g. usql) do not recognize this type and treated it as raw bytes, displaying Base64 or failing when the row encoder tried to read UUIDs as byte slices.

Fix: We downgraded the exposed column type for types.T_uuid to MYSQL_TYPE_VAR_STRING and updated the MySQL row encoding paths to handle UUIDs as strings under VAR_STRING. Added regression coverage to ensure UUIDs are encoded correctly with VAR_STRING and updated distributed SQL/result cases.

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