feat(appkit): stream agent model calls through the modular 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): stream agent model calls through the modular request seam

Serving, AI Gateway and Supervisor API streaming now go through the
workspace client's request() (modular transport, AppKit User-Agent, same
auth resolution incl. OBO token -> PAT) instead of the legacy
apiClient.request({ raw: true }). fromSupervisorApi resolves the host via
getHost() in place of config.ensureResolved().

The public supervisor WorkspaceClientLike stays exported. A legacy SDK
client with only apiClient.request (+ config.ensureResolved) is still
accepted through a deprecated fallback branch, so callers keep working.

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 (+22 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.3 MB (+22 KB)	455 KB (+8.5 KB)
Type declarations	461 KB (+5.1 KB)	167 KB (+1.8 KB)
Source maps	2.5 MB (+42 KB)	852 KB (+14 KB)
Other	11 KB	3.7 KB
Total	4.2 MB (+69 KB)	1.4 MB (+25 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 (+2.4 KB)	2.5 KB (+1 B)	107 KB (+2.4 KB)	external	339 KB (+7.1 KB)
./beta	100 KB (+1.7 KB)	488 B (+4 B)	101 KB (+1.7 KB)	external	302 KB (+4.4 KB)
./testing	43 KB (+1.4 KB)	32 KB (+35 B)	76 KB (+1.4 KB)	external	219 KB (+4.1 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	25 KB (+1.4 KB)	0 B	25 KB (+1.4 KB)	external	70 KB (+3.9 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	82 KB
./beta	stream-manager.js	initial	5.9 KB
./beta	modular.js	initial	4.6 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	657 B
./beta	index.js	initial	20 B
./beta	supervisor-api.js	lazy	193 B
./beta	databricks.js	lazy	180 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	25 KB

@databricks/appkit-ui

npm tarball (packed): 397 KB (+116 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	428 KB	146 KB
Type declarations	266 KB (+301 B)	98 KB (+129 B)
Source maps	852 KB	285 KB
CSS	16 KB	3.2 KB
Total	1.5 MB (+301 B)	533 KB (+129 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	49 KB	481 KB	1.3 MB	177 KB
./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