fix(BA-6259): drop stale top-level ui_type from install runtime variant preset fixture

[seedspirit]

Summary

• RuntimeVariantPresetRow folded the separate ui_type column into the ui_option JSONB, but the install seed fixture still carried a top-level "ui_type" key on each entry.
• This caused mgr fixture populate to fail with sqlalchemy.exc.CompileError: Unconsumed column names: ui_type.
• Removed the 29 stale top-level ui_type lines; ui_type remains correctly nested inside ui_option.

Test plan

• ./backend.ai mgr fixture populate src/ai/backend/install/fixtures/example-runtime-variant-presets.json → Done, no CompileError
• Delete all rows → confirm 0 → populate → 56 rows recreated (vllm 29 + sglang 27)
• Verified ui_option JSONB contains ui_type (e.g. dtype → "ui_type": "select")

Resolves BA-6259

[Copilot]

Pull request overview

This PR fixes installation fixture loading for runtime variant presets by removing a stale top-level ui_type field that no longer exists as a database column (it was folded into ui_option), preventing mgr fixture populate from failing with sqlalchemy.exc.CompileError: Unconsumed column names: ui_type.

Changes:

• Remove stale top-level ui_type keys from example-runtime-variant-presets.json entries while keeping ui_type nested under ui_option.
• Add a towncrier news fragment documenting the fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/ai/backend/install/fixtures/example-runtime-variant-presets.json	Drops top-level ui_type fields to match the current RuntimeVariantPresetRow schema and allow fixture population to succeed.
changes/11894.fix.md	Adds a changelog fragment describing the fixture populate CompileError fix.

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