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 (+21 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.3 MB (+21 KB)	455 KB (+8.0 KB)
Type declarations	460 KB (+4.8 KB)	167 KB (+1.8 KB)
Source maps	2.5 MB (+41 KB)	851 KB (+14 KB)
Other	11 KB	3.7 KB
Total	4.2 MB (+67 KB)	1.4 MB (+23 KB)

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

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	104 KB (+2.3 KB)	2.5 KB	107 KB (+2.3 KB)	external	338 KB (+6.8 KB)
./beta	100 KB (+1.5 KB)	486 B (+2 B)	101 KB (+1.5 KB)	external	302 KB (+4.3 KB)
./testing	43 KB (+1.4 KB)	32 KB (+37 B)	76 KB (+1.4 KB)	external	219 KB (+3.9 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	24 KB (+1.3 KB)	0 B	24 KB (+1.3 KB)	external	70 KB (+3.6 KB)

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	100 KB
.	utils.js	initial	4.6 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	81 KB
./beta	stream-manager.js	initial	5.9 KB
./beta	modular.js	initial	4.5 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	651 B
./beta	index.js	initial	20 B
./beta	supervisor-api.js	lazy	193 B
./beta	databricks.js	lazy	178 B
./beta	index.js	lazy	115 B
./testing	manifest.js	initial	30 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	24 KB

@databricks/appkit-ui

npm tarball (packed): 397 KB (+558 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 (+728 B)	98 KB (+349 B)
Source maps	853 KB (+314 B)	285 KB (+143 B)
CSS	16 KB	3.2 KB
Total	1.5 MB (+1.2 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 (+11 B)	49 KB	481 KB (+11 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 37949340134 -R databricks/appkit -n appkit-template-0.87.0-pr.c93a675-feat-modular-serving-639 -D appkit-pr-639 \
  && unzip -o "appkit-pr-639/appkit-template-0.87.0-pr.c93a675-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.