feat(appkit): move serving calls to the workspace client request seam

[MarioCadenas]

Part of the migration off @databricks/sdk-experimental onto the modular @databricks/sdk-* SDK (see #562). Base: feat/modular-sdk-integration.

Changes

feat(appkit): move serving calls to the workspace client request seam

Serving invoke, SSE streaming and the typegen OpenAPI fetch now go through
the facade's modular request() instead of the legacy SDK.

• invoke keeps the legacy query behavior: same request-field whitelist, raw
  JSON response (no unmarshal, so model-specific fields survive), the
  served-model-name header merge, and the same JSON parse error.
• streamPath uses client.request when present and keeps apiClient.request
  for caller-supplied legacy clients (public WorkspaceClientLike).
• The seam's toApiError now matches the legacy parseErrorFromResponse
  message/code, and Response carries the reason phrase it needs.

@databricks/sdk-modelserving is not used: it has no query method, and its
schema getter throws sdk-core ApiError, which the typegen 404/403 branches
can't read. The servingEndpoints accessor stays legacy to keep its public
type (query/getOpenApi) intact.

Draft: implemented by an agent; needs review and a deployed SP + OBO check before it's marked ready.

This pull request and its description were written by Isaac.

[github-actions]

📦 Bundle size report

Compared against bundle-size-baseline.json (main).

@databricks/appkit

npm tarball (packed): 1.2 MB (+27 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.3 MB (+26 KB)	457 KB (+9.7 KB)
Type declarations	462 KB (+6.8 KB)	168 KB (+2.6 KB)
Source maps	2.5 MB (+53 KB)	856 KB (+18 KB)
Other	11 KB	3.7 KB
Total	4.2 MB (+86 KB)	1.4 MB (+31 KB)

Per-entry composition (own code — deps external (as shipped))

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	105 KB (+3.2 KB)	2.5 KB	108 KB (+3.2 KB)	external	340 KB (+8.9 KB)
./beta	101 KB (+2.9 KB)	487 B (+2 B)	102 KB (+2.9 KB)	external	305 KB (+7.6 KB)
./testing	44 KB (+2.6 KB)	32 KB (+1 B)	77 KB (+2.6 KB)	external	222 KB (+7.0 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	26 KB (+2.5 KB)	0 B	26 KB (+2.5 KB)	external	73 KB (+6.6 KB)

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	101 KB
.	utils.js	initial	4.6 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	82 KB
./beta	stream-manager.js	initial	5.9 KB
./beta	modular.js	initial	5.7 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	650 B
./beta	index.js	initial	20 B
./beta	supervisor-api.js	lazy	193 B
./beta	databricks.js	lazy	179 B
./beta	index.js	lazy	115 B
./testing	manifest.js	initial	31 KB
./testing	index.js	initial	10 KB
./testing	wide-event-emitter.js	initial	2.9 KB
./testing	index.js	lazy	28 KB
./testing	remote-tunnel-manager.js	lazy	2.5 KB
./testing	utils.js	lazy	1.8 KB
./tsdown	index.js	initial	520 B
./type-generator	index.js	initial	26 KB

@databricks/appkit-ui

npm tarball (packed): 397 KB (+550 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	429 KB (+235 B)	147 KB (+108 B)
Type declarations	267 KB (+768 B)	98 KB (+349 B)
Source maps	853 KB (+314 B)	286 KB (+143 B)
CSS	16 KB	3.2 KB
Total	1.5 MB (+1.3 KB)	534 KB (+600 B)

Per-entry composition (consumer bundle — deps bundled, peerDeps external)

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
./js	5.3 KB	49 KB	55 KB	208 KB	14 KB
./js/beta	2.1 KB	0 B	2.1 KB	0 B	5.0 KB
./react	433 KB (+12 B)	49 KB	481 KB (+12 B)	1.3 MB	177 KB (+41 B)
./react/beta	4.9 KB	0 B	4.9 KB	0 B	12 KB

Chunks:

Entry	Chunk	Load	Size (gz)
./js	index.js	initial	5.2 KB
./js	chunk	initial	120 B
./js	apache-arrow	lazy	49 KB
./js/beta	beta.js	initial	2.1 KB
./react	index.js	initial	431 KB
./react	tslib	initial	2.1 KB
./react	apache-arrow	lazy	49 KB
./react/beta	beta.js	initial	4.9 KB

[github-actions]

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📦 Try this PR's app template

Scaffolds a new app from this PR's SDK build. Run it in any folder (requires the GitHub CLI — gh auth login — and the Databricks CLI):

gh run download 38918729089 -R databricks/appkit -n appkit-template-0.88.1-pr.984fadf-feat-modular-serving-639 -D appkit-pr-639 \
  && unzip -o "appkit-pr-639/appkit-template-0.88.1-pr.984fadf-feat-modular-serving-639.zip" -d "appkit-pr-639" \
  && databricks apps init --template "appkit-pr-639"

The template pins @databricks/appkit and @databricks/appkit-ui to tarballs built from this branch, so the scaffolded app runs against this PR's code.