fix token_endpoint_auth_method for public clients

[Rodriguespn]

⚠️ This PR is under development - Please do not merge until marked as ready for review.

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What kind of change does this PR introduce?

Bug fix

What is the current behavior?

When registering an OAuth client via dynamic client registration (POST /oauth/clients/register), the request fails with a NULL constraint violation:

ERROR: null value in column "token_endpoint_auth_method" of relation "oauth_clients" violates not-null constraint (SQLSTATE 23502)

This happens because:

1. The hosted database has the token_endpoint_auth_method column with a NOT NULL constraint (added via a hosted-only migration)
2. The OAuthServerClient model doesn't have the TokenEndpointAuthMethod field
3. registerOAuthServerClient never sets this field when creating a new client

What is the new behavior?

The token_endpoint_auth_method is now properly set during OAuth client registration:

• If explicitly provided in the request, that value is used
• For public clients, defaults to "none"
• For confidential clients, defaults to "client_secret_basic"

Additional context

This fix is required for MCP (Model Context Protocol) authentication to work with Supabase Auth as the OAuth provider. MCP clients like Claude Code check the /.well-known/oauth-authorization-server endpoint and if "none" is listed in token_endpoint_auth_methods_supported, they register as public clients with token_endpoint_auth_method=none.

Changes

• Added TokenEndpointAuthMethod field to OAuthServerClient struct
• Added DetermineTokenEndpointAuthMethod() function to determine the auth method based on explicit value or client type defaults
• Updated registerOAuthServerClient to set TokenEndpointAuthMethod when creating clients
• Removed the TODO workaround in oauthServerClientToResponse - now uses the stored value directly
• Added migration for open-source deployments to create the column and backfill existing data
• Added unit tests for DetermineTokenEndpointAuthMethod

[coveralls]

Pull Request Test Coverage Report for Build 20665607914

Details

• 24 of 24 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 68.804%

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Totals
Change from base Build 20599680500:	0.02%
Covered Lines:	14753
Relevant Lines:	21442

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💛 - Coveralls

[Rodriguespn]

Solved in this PR