feat(backend): Require discriminator value on graph save

[Pwuts]

If a node has a multi-credentials input (e.g. AI Text Generator block) but the discriminator value (e.g. model choice) is missing, the input can't be discriminated into a single-provider input. Discrimination into a single-provider input is necessary to make a graph-level credentials input for use in the Library.

Changes 🏗️

• feat(backend): Require discriminator fields to always have a value

• dx(frontend): Improve typing of discriminator stuff

• dx(frontend): Fix typing in NodeOneOfDiscriminatorField component

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:
  • Saving & running graphs with and without credentials works normally
  • Note: We don't have any blocks with a discriminator that doesn't have a default value, so currently I don't think it's possible to produce a case where this mechanism would be triggered.

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[deepsource-io]

Here's the code health analysis summary for commits d7077b5..ab4d9fe. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
JavaScript	✅ Success	❗ 2 occurences introduced 🎯 7 occurences resolved	View Check ↗
Python	✅ Success	❗ 1 occurence introduced 🎯 5 occurences resolved	View Check ↗

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[Pwuts]

Frontend checks are failing on the "add all blocks" test. 🤔

[kcze]

LGTM