refactor: EnumText for preferred_provider_type MessageChain, Banner

[tmimmanuel]

Summary

• Replace sa.String with EnumText(ProviderType) for TenantPreferredModelProvider.preferred_provider_type
• Replace sa.String with EnumText(MessageChainType) for MessageChain.type
• Replace sa.String with EnumText(BannerStatus) for ExporleBanner.status
• Remove redundant ProviderType.value_of() and .value conversions

Continues #33294 (provider + model domain, part 2)

Details

Model	Column	Before	After
TenantPreferredModelProvider.preferred_provider_type	String(40)	EnumText(ProviderType)
MessageChain.type	String(255)	EnumText(MessageChainType)
ExporleBanner.status	String(255)	EnumText(BannerStatus)

With EnumText, the column now returns the enum directly — no need for ProviderType.value_of() to parse the raw string on read, or .value to extract the string on write.

Test plan

• basedpyright passes (0 errors on changed files)
• 123 unit tests pass (provider models + provider configuration + banner)
• CI lint + type-check + full test suite

🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors several SQLAlchemy model columns to use EnumText instead of sa.String for enum-backed fields. This change enhances type safety, improves code readability by allowing direct enum usage, and eliminates the need for manual string-to-enum and enum-to-string conversions when interacting with the database.

Highlights

• TenantPreferredModelProvider.preferred_provider_type: Replaced sa.String with EnumText(ProviderType) for the preferred_provider_type column.
• MessageChain.type: Replaced sa.String with EnumText(MessageChainType) for the type column.
• ExporleBanner.status: Replaced sa.String with EnumText(BannerStatus) for the status column.
• Enum value conversions: Removed redundant ProviderType.value_of() and .value conversions, simplifying enum handling.

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[github-actions]

Pyrefly Diff

No changes detected.

[gemini-code-assist]

Code Review

This pull request effectively refactors the codebase to leverage EnumText for TenantPreferredModelProvider.preferred_provider_type, MessageChain.type, and ExporleBanner.status. This change significantly improves type safety and readability by allowing direct use of enum members instead of string literals, and correctly removes redundant .value_of() and .value conversions. The changes are well-implemented across models, controllers, and tests, demonstrating a thorough approach to the refactoring. All modifications align with the stated goals of the pull request and enhance the maintainability of the code.

[github-actions]

Pyrefly Diff

No changes detected.

[tmimmanuel]

@asukaminato0721 Please review my PR. Thanks.

[tmimmanuel]

@asukaminato0721 Thank you.