feat(appkit): migrate files to modular @databricks/sdk-files

[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): migrate files to modular @databricks/sdk-files

Swap the facade's files accessor to a lazily built FilesClient
(sdk-files 0.52.0, accepts sdk-core/auth/options 0.51.0).

• list/download/createDirectory/delete use listDirectoryContentsIter,
  downloadFile, createDirectory, deleteFile; results are mapped back to
  the legacy snake_case / header-keyed shapes.
• metadata uses a raw HEAD via request(): the generated getFileMetadata
  parses the empty HEAD body and drops the metadata headers.
• Modular ApiError (httpStatusCode) is converted to the wrapper ApiError
  so exists() and the plugin's 4xx mapping keep working.
• DirectoryEntry / DownloadResponse are now AppKit-owned types with the
  same shape.

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 (+19 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	1.2 MB (+19 KB)	454 KB (+7.1 KB)
Type declarations	460 KB (+4.5 KB)	167 KB (+1.7 KB)
Source maps	2.5 MB (+37 KB)	849 KB (+12 KB)
Other	11 KB	3.7 KB
Total	4.2 MB (+60 KB)	1.4 MB (+21 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 (+1.9 KB)	2.5 KB	106 KB (+1.9 KB)	external	337 KB (+5.6 KB)
./beta	100 KB (+1.4 KB)	485 B (+1 B)	100 KB (+1.4 KB)	external	301 KB (+3.8 KB)
./testing	43 KB (+1.2 KB)	32 KB (+3 B)	75 KB (+1.2 KB)	external	218 KB (+3.6 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	24 KB (+1.1 KB)	0 B	24 KB (+1.1 KB)	external	70 KB (+3.2 KB)

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	99 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	4.4 KB
./beta	service-context.js	initial	4.2 KB
./beta	databricks.js	initial	3.3 KB
./beta	client.js	initial	596 B
./beta	index.js	initial	20 B
./beta	supervisor-api.js	lazy	192 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 37949341119 -R databricks/appkit -n appkit-template-0.87.0-pr.efb955c-feat-modular-files-638 -D appkit-pr-638 \
  && unzip -o "appkit-pr-638/appkit-template-0.87.0-pr.efb955c-feat-modular-files-638.zip" -d "appkit-pr-638" \
  && databricks apps init --template "appkit-pr-638"

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.