refactor(shared): remove the legacy @databricks/sdk-experimental SDK

[MarioCadenas]

Summary

Final layer of the modular SDK stack: removes the legacy @databricks/sdk-experimental SDK. Every service already runs on the modular @databricks/sdk-* clients; this deletes the legacy client and drops the dependency.

Changes

• Removed: legacy.ts, toLegacyWorkspaceClient(), and the facade's servingEndpoints / config / apiClient accessors. Client options moved to workspace-client/options.ts.
• Last runtime uses moved to the modular client:
  • the persistent cache now hands the modular client to Lakebase;
  • appkit doctor passes the facade to Lakebase instead of a hand-built legacy shape;
  • boot auth errors map the modular SDK's credential errors (isAuthConfigError) to ConfigurationError, replacing the legacy ConfigError;
  • the AbortSignal cancellation bridge no longer needs the SDK's Context class.
• Replaced: ApiError is AppKit's own class (same fields; instanceof also matches modular SDK errors). Local types replace the legacy serving / sql namespaces for raw JSON.
• Dependency dropped from shared, appkit, the app template and lockfiles, plus knip, CLAUDE.md, the oxlint boundary message and the testing docs. The API reference is regenerated in its own commit.

Kept on purpose: agents and @databricks/lakebase still accept a caller-supplied legacy-shaped client. That support is structural (no SDK import), and lakebase is published on its own.

Notes

• @databricks/sdk-experimental@0.15.0 stays in the lockfile as a transitive dependency of @mlflow/core@0.4.0 (optional peer for agent tracing).
• The template lockfiles still list the legacy SDK under the published appkit@0.88.1 / lakebase@0.6.0 packages; that goes away with the next release and template sync.

Test plan

• pnpm build, pnpm -r typecheck, pnpm check, pnpm docs:build
• Unit tests: 5676 passed
• Deployed e2e app on the new template build

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 (+28 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.3 MB (+28 KB)	457 KB (+10 KB)
Type declarations	464 KB (+8.0 KB)	168 KB (+2.8 KB)
Source maps	2.5 MB (+57 KB)	855 KB (+18 KB)
Other	11 KB	3.7 KB
Total	4.2 MB (+93 KB)	1.4 MB (+30 KB)

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

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	106 KB (+3.9 KB)	2.5 KB	108 KB (+3.9 KB)	external	343 KB (+12 KB)
./beta	102 KB (+3.8 KB)	487 B (+2 B)	103 KB (+3.8 KB)	external	308 KB (+11 KB)
./testing	45 KB (+3.1 KB)	32 KB (+1 B)	77 KB (+3.1 KB)	external	223 KB (+8.7 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	26 KB (+2.8 KB)	0 B	26 KB (+2.8 KB)	external	74 KB (+7.9 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	modular.js	initial	6.0 KB
./beta	stream-manager.js	initial	5.9 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	649 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	32 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 (+497 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 (+983 B)	98 KB (+225 B)
Source maps	853 KB (+314 B)	286 KB (+143 B)
CSS	16 KB	3.2 KB
Total	1.5 MB (+1.5 KB)	534 KB (+476 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 38924102701 -R databricks/appkit -n appkit-template-0.88.1-pr.a7503ac-feat-modular-remove-legacy-sdk-673 -D appkit-pr-673 \
  && unzip -o "appkit-pr-673/appkit-template-0.88.1-pr.a7503ac-feat-modular-remove-legacy-sdk-673.zip" -d "appkit-pr-673" \
  && databricks apps init --template "appkit-pr-673"

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.