Give exported connection files an explicit wire-format type

[kmcginnes]

Description

Slice 1 of retiring the legacy RawConfiguration "god object". The exported connection-file format (saveConfigurationToFile → isValidConfigurationFile on import) borrowed the in-memory configuration shape via Pick<ConfigurationContextProps, "id"|"displayLabel"|"connection"|"schema">. That borrowing is what keeps the unused in-memory RawConfiguration.schema field alive, so it blocks the later slices.

This gives the on-disk format its own honest type, ExportedConnectionFile, decoupled from the in-memory and persisted shapes so the wire format can evolve independently. Schema is required (not optional), and lastUpdate is typed as the ISO string it actually is on disk.

While here, the hand-rolled validator is replaced with a Zod schema (matching the sibling exportedGraph.ts wire format). The old imperative version asserted the stronger type but never checked that schema.vertices / schema.edges were arrays before iterating them, so a malformed file threw an uncaught TypeError instead of being rejected with the "Invalid File" toast. The Zod schema enforces the shape declaratively and stays lenient about unknown/legacy fields, so styling and prefix __matches still pass through on import (the import hook reads the original parsed object, not the schema's output).

Suggested reading order:

1. isValidConfigurationFile.ts — the new ExportedConnectionFile type and the Zod validator that replaces the throw-prone imperative loops (core change).
2. saveConfigurationToFile.ts — now explicitly typed as ExportedConnectionFile.
3. useImportConnectionFile.ts — drops now-redundant optional chaining on the guaranteed schema.
4. Tests + CONTEXT.md glossary entry.

One accepted, tiny behavior change: a connection-less export now emits connection.url: "" instead of omitting url (forced by the strict type; aligns the writer with the validator, which already requires url). This is a not-actually-reachable state and is pinned by a test noting it should disappear in a later slice.

This is the first of three stacked PRs. Slice 2 removes RawConfiguration.schema; slice 3 migrates the storage type.

Validation

• pnpm checks passes (lint, format, types).
• pnpm test passes — full suite, 1793 tests.
• New tests pin the malformed-schema rejection (previously threw), the save→validate round-trip, and the url: "" asymmetry. Backward-compat pins for legacy imports and backups stay green.

Related Issues

• Stacked on Remove the dead config schema leg from the configuration merge #1821 (base branch: `simplify-configuration-merge`)

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.