feat(gotrue): convert OAuthProvider from enum to class for custom provider support

[grdsdev]

Summary

• Converts OAuthProvider from a plain Dart enum to a final class, enabling arbitrary provider strings at runtime
• Adds a regression test covering construction, URL generation, equality, and values enumeration
• All existing call sites (OAuthProvider.google, .snakeCase, == OAuthProvider.google) continue to work unchanged

Root Cause

OAuthProvider was declared as a Dart enum. The Supabase docs show:

await supabase.auth.signInWithOAuth(OAuthProvider('custom:my-provider'));

But calling a generative enum constructor at runtime is illegal in Dart, producing:

Generative enum constructors can only be used to create an enum constant.

Fixes: #1337

Changes

• packages/gotrue/lib/src/types/types.dart: Replace enum OAuthProvider with final class OAuthProvider. Built-in providers become static const fields storing pre-converted snake_case wire names (linkedinOidc → 'linkedin_oidc', slackOidc → 'slack_oidc'). Adds snakeCase getter, ==/hashCode, and static const List<OAuthProvider> values.
• packages/gotrue/test/custom_oauth_provider_test.dart: New regression test (6 cases).

Backward Compatibility

Usage	Status
OAuthProvider.google / all static constants	✅ Unchanged
provider == OAuthProvider.google	✅ Works via == override
provider.snakeCase	✅ Works via class getter
OAuthProvider.values	✅ Preserved as static const list
switch (provider) exhaustiveness	⚠️ No longer compiler-enforced — users need a default case
linkedinOidc.name / slackOidc.name	⚠️ Changes from 'linkedinOidc' → 'linkedin_oidc' (internal enum name, not the API wire value)

Testing

dart test test/custom_oauth_provider_test.dart
+6: All tests passed!

dart analyze packages/gotrue/lib/ packages/supabase_flutter/lib/
No issues found!

Linear

SDK-794

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🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the GoTrue Dart API to support runtime-defined OAuth providers by replacing OAuthProvider from a Dart enum to a final class, aligning the SDK with documented usage for custom providers.

Changes:

• Replaces enum OAuthProvider with a final class OAuthProvider that accepts arbitrary provider strings and preserves existing static constants.
• Adds value-based equality/hashCode and a values list for built-in provider enumeration.
• Introduces a regression test validating custom provider construction, URL generation, equality, and values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
packages/gotrue/lib/src/types/types.dart	Converts OAuthProvider to a class with const built-ins, values, and value-based semantics.
packages/gotrue/test/custom_oauth_provider_test.dart	Adds regression tests for custom provider support and compatibility behaviors.

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