Use the const value when a literal branch is selected in a schema form

[devin-ai-integration]

closes #22909

This PR makes the React schema form use the const value of a union branch.

Pydantic gives a Literal in a union an anyOf definition with a const and no default:

{
  "anyOf": [
    { "items": { "type": "string" }, "type": "array" },
    { "const": "bar", "type": "string" },
    { "type": "null" }
  ]
}

The user cannot type the value of a const, and the form did not supply it. When the user selected the literal branch, the value stayed empty. The form did not show an error, and it sent null for the parameter. Thus the user could not give the literal value, and the run got the wrong value.

Details

The form now does two things:

• When the user selects a branch that has no value, the form emits the const of that branch.
• Branch selection prefers a definition whose const is equal to the value, before a definition that only has the same type as the value. A union with Literal["bar"] and str in it now shows the literal branch for the value "bar", and the str branch for other strings.

Tests:

• ui-v2/src/components/schemas/schema-form-input-any-of.test.tsx — the form emits "bar" when the user selects the literal branch of list[str] | Literal["bar"] | None.
• ui-v2/src/components/schemas/utilities/getIndexForAnyOfPropertyValue.test.ts — index resolution prefers the const match, and keeps the type match when no const is equal to the value.

The behavior is also verified in a browser against a local server. See the comment below.

The same defect is in the Vue schema form. The change for the V1 UI is in PrefectHQ/prefect-ui-library#3133, and the change for the Cloud UI is in PrefectHQ/nebula-ui#7070.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this is a complex change, a maintainer has confirmed the proposed approach on the linked issue.
• If this pull request adds or changes functionality, it includes tests or explains why tests are not needed.
• If this pull request changes user-facing behavior, it updates documentation or explains why documentation is not needed.
  • The change corrects form behavior only, so no documentation change is necessary.
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Link to Devin session: https://app.devin.ai/sessions/c7d997cb25b0431786119152836c2251

[devin-ai-integration]

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[devin-ai-integration]

Runtime verification of the deployment Run form (ui-v2)

Tested against a local Prefect server with the reporter's deployment (foo: list[str] | None | Literal["bar"], deployed with parameters={"foo": None}), comparing this branch (:5173) against its base commit in a separate worktree (:5174).

After (this branch): the str branch fills with bar, and the run gets {"foo": "bar"}

Before (base commit): the literal branch stays empty, and the form sends null

The form does not show a validation error on the base commit. Submit is accepted, and the run gets {"foo": null}. The user cannot give the literal value, and the wrong value goes to the run.

Regression: the array branch and the null branch continue to work

• list branch, then "Add item", then hello gives run parameters {"foo": ["hello"]}
• str branch, then back to None, gives run parameters {"foo": null}

Parameters of the created runs, from GET /api/flow_runs/filter:

{'foo': 'bar'}       # this branch, str branch
{'foo': ['hello']}   # this branch, list branch
{'foo': None}        # this branch, None branch
{'foo': None}        # base commit, str branch selected (the defect)

Not covered: the V1 Vue form (PrefectHQ/prefect-ui-library#3133) was not exercised in a browser.

Verified by Devin — https://app.devin.ai/sessions/c7d997cb25b0431786119152836c2251