feat(cache): config-server cache write backend

[steiler]

Ticket 07 of the #460 + #471 PR-split reorg
(see graph.md/index.md in the tracking gist for the full stack). Base:
cache-read-backend (#496).

Scope

Real IntentWriter for Cache.Type: config-server: IntentModify/IntentDelete write
through to config-server's ConfigSnapshotService synchronously at apply time (ADR 0003),
closing the ghost-intent race where a deleted intent stayed last-applied until the next
best-effort refresh.

Cross-repo dependency

Depends on sdcio/sdc-protos#135, which renames
ConfigReadService to ConfigSnapshotService and adds Modify/Delete RPCs — a rebase of
the dangling last-applied-snapshot-write-at-apply branch's 5a14ea0 onto this stack's
cache-read-backend tip (sdc-protos#134). go.mod is pinned to that rebased commit; swap for
a tagged release once #123/#134/#135 merge.

What shipped

• ConfigServerCache gains a configserver.ConfigSnapshotClient port (read+write) and
  implements IntentWriter directly — InstanceIntentModify/InstanceIntentDelete write
  through synchronously, at the same moment TransactionSet's apply loop calls them, matching
  Cache.Type: local's write-at-apply timing.
• New tree-to-JSON flattener (configsnapshot.DocumentFromIntent) converts an applied
  tree_persist.Intent into the flat Document shape the read path already understands, for
  the wire Modify payload.
• Fixed the log-only IntentDelete failure in lowlevelTransactionSet: a failed delete-write
  now hard-fails the transaction, matching IntentModify's existing behavior (ADR 0003's
  asymmetry fix).
• GRPCConfigClient/FakeConfigSnapshotClient implement the collapsed ConfigSnapshotClient
  port (renamed from the read-only GRPCConfigReader/FakeLocalConfigReader); dropped the
  half-split LocalConfigReader/LocalConfigWriter interfaces entirely once nothing was typed
  against either alone.
• Extracted the last-applied Document interchange (flatten, blob merge, NewImportAdapter) into
  its own pkg/cache/configsnapshot package, so Modify→Get round-trips live in one place;
  pkg/cache/configserver now owns only the remote ConfigSnapshotClient port.
• Colocated the config-server Namespaced-name codec (splitDatastoreName/lookupConfigName)
  next to ConfigServerCache in pkg/cache/configserver_names.go.
• Removed the now-dead noopIntentWriter and the dual NewConfigServerClient wrapper —
  Server.createConfigServerCacheClient composes cache.NewConfigServerCache directly.
• Tests: seam-level modify/delete round trips, malformed-datastore-name coverage for the new
  write callers, and datastore-level regression tests for delete-failure hard-fail and
  rollback-restores-a-deleted-intent (content, not just name).
• Small independent cleanup: replaced the hardcoded "unknown" owner literals across the four
  YANG validators with a new types.UnknownOwner constant (no behavior change).
• Committed ADR 0003 (pkg/cache/docs/adr/0003-config-server-write-path-real-last-applied-writes.md)
  and the CONTEXT.md Last-applied/ConfigSnapshotService/Namespaced-name glossary updates.

Adapted from the source branch

• Cherry-picked 52d842e/7f4f4da/22899e2/6f0d373/b3f52c6 from
  origin/config-server-cache-backend, plus ticket 06's deferred follow-ups 706ecb6/2e8ffc6,
  re-resolving each against this stack's actual, narrower type shapes (ticket 05's
  IntentAdapter/ImportConfigAdapter split, ticket 06's pkg/cache/configserver package
  layout) rather than the source branch's pre-split/pre-relocation originals.
• Reused the existing buildFixtureIntent test helper from sync_test.go instead of
  duplicating it, and fixed two new IntentGetAll mock expectations to the
  importer.IntentAdapter channel type.
• Dropped ticket 06's now-superseded TestConfigServerCache_WritesAreNoOps /
  TestCreateConfigServerCacheClient_WritesAreNoOps (writes are real now, not no-ops).
• Deliberately not cherry-picked: 10652b4/b3ca60d/f0c91db's ghost-exclusion
  mechanism (net-cancelling in the source history; never added to this stack in the first
  place — ticket 05 already skipped 10652b4) and 181394c (superseded — ticket 06 already
  built InstanceCreate's empty-Intent seeding in correctly). b27ab78/15b7efd (net-zero
  lock-retry pair) and 2c28a90/69aab56/6c1bf7d (local-dev cruft / empty CI retriggers)
  also dropped, consistent with prior tickets.
• Folded only the pkg/cache/-relevant sliver of 67f4bdf (a dead docs/architecture.md
  reference in ADR 0001) rather than that commit's much larger, unrelated repo-wide docs/agent-
  process reorg.
• 0525b9d (CI-failure investigation notes for a fix that landed in config-server/
  integration-tests, not here) dropped entirely, per the ticket's own note.
• Dropped .scratch/ planning artifacts, consistent with prior tickets.

Verification

go build ./..., go vet ./..., go test ./... (full repo), and golangci-lint run ./...
all pass.

[codecov]

Codecov Report

❌ Patch coverage is 66.91729% with 44 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/cache/configserver/grpc.go	13.33%	26 Missing ⚠️
pkg/cache/configsnapshot/document_export.go	77.77%	5 Missing and 5 partials ⚠️
pkg/cache/configserver.go	90.90%	1 Missing and 1 partial ⚠️
.../tree/ops/validation/validation_entry_mandatory.go	0.00%	2 Missing ⚠️
pkg/tree/ops/validation/validation_entry_must.go	0.00%	2 Missing ⚠️
...kg/tree/ops/validation/validation_entry_leafref.go	50.00%	1 Missing ⚠️
...ps/validation/validation_entry_min_max_elements.go	0.00%	1 Missing ⚠️

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