fix: persist auth method switch from OAuth to Bearer/none in MCP catalog

[ozeranskii]

Summary

• Fix frontend transformFormToApiData using undefined instead of null when clearing oauthConfig - JSON.stringify() strips undefined, so the backend never receives the field and the old OAuth config stays in the database
• Add backend cleanup of orphaned OAuth client secret (clientSecretId) when oauthConfig is explicitly set to null in the PUT handler
• Add 5 frontend tests for transformFormToApiData covering all auth methods and JSON serialization
• Add 2 backend tests for OAuth-to-Bearer and OAuth-to-none switching at the model level

Closes #3076

[London-Cat]

📊 Reputation Summary

User	Rep	Pull Requests	Activity	Assigned	Core Reactions
ozeranskii	⚡ 3	0✅ 1🔄 0❌	0 issues, 0 comments	0	—
CLAassistant	⚡ 0	0✅ 0🔄 0❌	0 issues, 50 comments	0	—

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[CLAassistant]

All committers have signed the CLA.