fix(frontend): Unbreak credentials input on single-provider blocks

[Pwuts]

• Patch for breakage from feat(platform): Simplify Credentials UX #8524
• Resolves Fix broken credentials input on single-provider blocks #8635

Changes 🏗️

• fix(frontend): Fix type mismatch of CredentialsField schema between frontend and backend

  • Fix usages of credentialsSchema.credentials_provider

• refactor(backend): Create CredentialsFieldSchemaExtra model in backend so it can be mirrored directly in frontend

  • Add check to enforce multi-provider CredentialsField always has discriminator

• dx: Add type checking shortcut yarn type-check / npm run type-check for frontend

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
  • Save and run a graph with a single-provider block
  • Save and run a graph with a multi-provider block

Example test plan

• Create from scratch and execute an agent with at least 3 blocks
• Import an agent from file upload, and confirm it executes correctly
• Upload agent to marketplace
• Import an agent from marketplace and confirm it executes correctly
• Edit an agent from monitor, and confirm it executes correctly

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