fix(metadata-protocol): the 422 issues[] descends invalid_key / invalid_element, so a rejected record key arrives with the rule it broke (#8817)

* fix(metadata-protocol): descend invalid_key / invalid_element in the 422 issues[] (#8783)

zodIssuesToMetadataIssues expanded invalid_union only, so a z.record/z.map
key rejection reached Studio as the bare wrapper "Invalid key in record"
while the rule it broke sat one level down in issue.issues — the shape #4971
named, and the one both packages/spec walks have descended since #5389.

ObjectSchema.fields is a constrained-key record carrying the snake_case rule
and object is in the builtin registry, so this was the commonest authoring
mistake on the most-authored metadata type.

The descent is additive and matches what the two spec walks were measured to
emit for the card's own repro: the wrapper entry stays at index 0, the
detail joins it on the same slot.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XeQRiAa7vYRVX5Fog7Zby8

* test(metadata-protocol): pin the container descent across all three walks + changeset (#8783)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XeQRiAa7vYRVX5Fog7Zby8

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: hotlong

.changeset/metadata-422-container-issue-descent.md

@@ -0,0 +1,60 @@
+---
+"@objectstack/metadata-protocol": patch
+---
+
+fix(metadata): the `422 INVALID_METADATA` envelope descends `invalid_key` / `invalid_element`, so a rejected record key arrives with the rule it broke (#8783)
+
+Zod raises a `z.record` / `z.map` **key** rejection as `invalid_key` and a
+`z.map` **element** rejection as `invalid_element`, and in both cases the
+issue's own `message` is a bare wrapper — `"Invalid key in record"` — with the
+real diagnosis one level down in `issue.issues`. That is structurally the
+`invalid_union` shape #4971 named: the prescription is produced and then
+dropped by a walk that reads only the top level.
+
+Both `packages/spec` walks learned to descend those codes in #5389.
+`zodIssuesToMetadataIssues` — the walk behind `saveMetaItem`'s 422 (#5364) and
+the read path's diagnostics (#5598) — expanded `invalid_union` only, so it
+stopped at the wrapper. Three walks over one `safeParse`, two of them reaching
+the prescription and the Studio-facing one not.
+
+**It was reachable from ordinary authored metadata, not synthetic.**
+`ObjectSchema.fields` is a record whose KEY schema carries the snake_case rule
+(`spec/src/data/object.zod.ts`), and `object` is in the builtin
+`getMetadataTypeSchema` registry. So the commonest authoring mistake on the
+most-authored metadata type — writing `firstName` for a field key, which is
+exactly what an agent coming from JS naming writes — produced:
+
+```
+{ path: 'fields.firstName', code: 'invalid_key', message: 'Invalid key in record' }
+```
+
+The author was told a key was invalid and never told what a valid one looks
+like, so the next move was to guess. The declared message existed and was
+correct; it just did not reach anyone. Now the same save answers:
+
+```
+{ path: 'fields.firstName', code: 'invalid_key',    message: 'Invalid key in record' }
+{ path: 'fields.firstName', code: 'invalid_format', message: 'Field names must be lowercase snake_case (e.g., "first_name", …)' }
+```
+
+**Additive, and matched to the walks that already worked** rather than chosen.
+The other two were measured over the card's own repro first: `formatZodIssue`
+prints the wrapper line then the indented detail, and `zodIssuesToFields` emits
+the `invalid_shape` wrapper entry then the detail entry. So the wrapper stays at
+index 0 — it is the only entry naming the slot the client sent, and Studio's
+designer keys on it — and the detail joins it on the same path. No entry that
+shipped before is removed or renumbered.
+
+**Targeted, not a widened walk.** Only the two container codes open the descent;
+an `issues` array hanging off any other code is still ignored, `invalid_union`
+still expands through the unchanged ranking, and the nesting bound now covers
+both descents at the same depth of 3. Container issues are deliberately *not*
+ranked the way union branches are: a union's branches are competing candidates,
+while a container has one inner schema, so every issue it raised is a true
+statement about the value.
+
+The verdict is unchanged in every case — this moves what a refusal *says*, never
+whether it is one. `union-branch-policy.cross-package-parity.test.ts` gains a §5
+comparing the container descent across all three walks; its §1 (the policy is
+not publicly exported from `@objectstack/spec`, so this package must run its own
+copy) is untouched, and no export was added.