Remove the schema field from the stored configuration type

[kmcginnes]

Description

Slice 2 of the configuration-model simplification (Option B). Slice 1 (#1822) has merged, so this now targets main directly.

A Connection's data has three shapes that were being conflated through one RawConfiguration type: the on-disk export file, the in-memory merged shape the UI reads, and the IndexedDB storage shape. This slice separates the storage shape from the in-memory merged shape:

• RawConfiguration (persisted / configurationAtom / IndexedDB) is now { id, displayLabel?, connection? } — no schema. The schema lives only in schemaAtom; nothing read it back off the stored config.
• New MergedConfiguration = RawConfiguration & { schema } is the output of mergeConfiguration — the schema is required, since the merge always builds it. ConfigurationContextProps extends it.

Dropping the dead schema field from storage and tightening it to required on the merged shape also removes the defensive ?.schema?. chaining that was scattered across every consumer.

How to read

1. core/ConfigurationProvider/types.ts — the type split (start here).
2. core/StateProvider/configuration.ts + core/ConfigurationProvider/useConfiguration.ts — mergeConfiguration return type and the consumers losing their optional chains.
3. utils/saveConfigurationToFile.ts — reads schema off the merged shape; vertices/edges defaulting drops out.
4. Tests — the rest. saveConfigurationToFile.test.ts gains a small makeConfig() factory that removes repeated fixture boilerplate.

Backward compatibility

Removing a field from a persisted type. Legacy IndexedDB blobs and exported files may still embed a schema on the config; three regression pins guard the ends and were each mutation-tested:

• storage round-trip carries a stray embedded schema through untouched (localDb.test.ts)
• import never writes the schema back into the config entry (useImportConnectionFile.test.tsx)
• the merge sources its schema only from the active schema, never from one embedded on the stored config (configuration.test.ts)

Validation

• pnpm checks clean; pnpm test green (rebased onto current main).
• All three backward-compat pins mutation-tested (broke source, confirmed each fails, reverted).
• The required-schema invariant is enforced at compile time — renaming the merge's schema key fails typecheck.

Related Issues

• Closes Remove the dead schema field from the stored configuration type #1842
• Related to Fully parse the exported connection file with a Zod wire-format schema #1822

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.