fix(dropdown): keep query-sourced selections while the options query loads

[alecrt]

Summary

Prevents single-select and multi-select dropdowns from prematurely clearing query-sourced selections when static dropdown_option children (e.g. an "All" option) are present, ensuring URL-hydrated filter values and initial_value selections survive initial page hydration while options are loading.

Problem / Motivation

A dropdown often mixes static dropdown_option children (typically an "All" or default entry) with dynamic options populated from data=:

<Dropdown data={categories} id="category" value_column="code" initial_value="all">
    <DropdownOption value="all" label="All Categories" />
</Dropdown>

Currently, isDropdownValueValid (core/src/user-components/tags/dropdown/validation.ts) switches validation on as soon as static options exist (hasStaticOptions). Because the options query has not resolved on first paint, availableValues only contains the static options.

What went wrong:

• Any query-sourced selection (such as a link opened with ?category=electronics) fails validation against the static option set alone and is cleared on initial mount.
• The URL filter writer subsequently detects the empty value and strips the query parameter from the browser URL, destroying shareable/bookmarkable links.
• initial_value suffers the same failure whenever it refers to a query-sourced value.
• Conversely, dropdowns with no static children keep their query-sourced values as expected, which makes this bug particularly confusing and difficult to diagnose.

Solution

• Introduce an explicit optionsQueryPending state (queryConfig !== undefined && optionsQuery.result === undefined) indicating that an options query is configured but has not yet produced a result.
• While optionsQueryPending is true, isDropdownValueValid preserves the current selection regardless of static options.
• Once the options query settles, standard validation resumes immediately, ensuring cascading dropdowns still clear genuinely stale selections when dependent parameters change.
• A query that encounters an error is treated conservatively in the same way, consistent with Evidence's existing handling for empty query results: preserving a selection that cannot currently be checked is safer than clearing a valid one.
• Dropdowns without an options query have no pending state and continue validating against static options exactly as before.

Testing

• Added unit tests in core/src/user-components/tags/dropdown/validation.test.ts:
  • keeps a query-sourced value while the options query is still pending, despite static options
  • validates again as soon as the options query settles
  • leaves dropdowns without a query untouched: no pending state, static options still validate
• Ran unit tests via Vitest: 10/10 passed cleanly.
• Ran type checks via pnpm --filter @evidence/core check: 0 errors.

Checklist

• Changes are confined to core/** (no files at repository root).
• Follows project code style and conventions.
• Commit message follows Conventional Commits format.

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