[WIP] reimplement UC Volumes filer on the files/v2 client

[renaudhartert-db]

What

The fs command's UC Volumes filer now runs on the new sdk-go/files/v2 client instead of hand-written Files API calls. This filer is shared, so bundle library uploads to Volumes go through the same path.

files/v2 is not published to the Databricks Go proxy yet, so it is vendored under libs/tmp/ together with the two unpublished sibling modules it needs (auth and options). The already-published sdk-go/core module stays a normal dependency and is not copied. libs/tmp/README.md records where the copy came from and how to remove it once the module is published (rewrite the import prefix back and delete the directory).

files/v2 ships its own multipart/resumable upload engine, so the CLI's separate libs/upload engine is removed. The DATABRICKS_EXPERIMENTAL_MULTIPART_UPLOAD flag still gates whether large Volumes uploads are split into parts; it now selects multipart-vs-single-shot within files/v2 rather than switching engines. Default behavior (single-shot PUT) is unchanged.

Why

Groundwork for the CLI to consume sdk-go/files/v2 as a real dependency once it is published. Vendoring lets us build and dogfood the client, and its own upload engine replaces the CLI-side one so there is a single implementation to maintain.

Notable in the diff

• libs/tmp/ is a verbatim vendored copy. The only edits are the import-path rewrite and two fixes to the generated client (empty response bodies and HTTP method casing); both are upstream generator bugs and are listed in libs/tmp/README.md. It is excluded from lint/format in .golangci.yaml so it stays byte-for-byte with upstream.
• libs/filer/files_client.go is rewritten onto *files.Client. NewFilesClient gains a ctx parameter (threaded through its callers).
• Shared code: the Volumes filer is used by both fs and bundle/libraries, so bundle Volumes uploads are affected. .github/OWNERS retargets the removed /libs/upload/ entry to /libs/tmp/. go.mod/NOTICE gain sdk-go/core as a direct dependency.

Out of scope

• Local, DBFS, and workspace filers are unchanged; files/v2 only backs the UC Volumes path.
• Publishing sdk-go/files/v2 is separate work; libs/tmp/ is the interim copy.

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/bundle/ - needs approval

Files: bundle/libraries/filer_volume.go
Suggested: @pietern
Also eligible: @denik, @shreyas-goenka, @janniklasrose, @andrewnester, @anton-107, @lennartkats-db

/cmd/fs/ - needs approval

4 files changed
Suggested: @simonfaltum
Also eligible: @Divyansh-db, @hectorcast-db, @parthban-db, @tanmay-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/experimental/air/ - needs approval

Files: experimental/air/cmd/snapshot.go
Suggested: @riddhibhagwat-db
Also eligible: @apeforest, @bfontain, @lu-wang-dl, @panchalhp-db, @vinchenzo-db, @maggiewang-db, @ben-hansen-db, @pardis-beikzadeh-db

/integration/ - needs approval

Files: integration/cmd/fs/helpers_test.go, integration/libs/filer/helpers_test.go
Suggested: @simonfaltum
Also eligible: @Divyansh-db, @hectorcast-db, @parthban-db, @tanmay-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/internal/ - needs approval

Files: internal/build/notice_test.go
Suggested: @simonfaltum
Also eligible: @Divyansh-db, @hectorcast-db, @parthban-db, @tanmay-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

/libs/filer/ - needs approval

Files: libs/filer/files_client.go, libs/filer/files_client_auth.go, libs/filer/files_client_test.go
Suggested: @simonfaltum
Also eligible: @Divyansh-db, @hectorcast-db, @parthban-db, @tanmay-db, @tejaskochar-db, @mihaimitrea-db, @chrisst, @rauchy

General files (require maintainer)

44 files changed
Based on git history:

• @pietern -- recent work in ./, libs/filer/, libs/testserver/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @janniklasrose, @lennartkats-db) can approve all areas.
See OWNERS for ownership rules.}

[eng-dev-ecosystem-bot]

Integration test report

Commit: 39fe347

Run: 29899233743

	Env	🟨​KNOWN	🔄​flaky	💚​RECOVERED	🙈​SKIP	✅​pass	🙈​skip	Time
💚​	aws linux			4	4	314	1052	5:10
💚​	aws windows			4	4	316	1050	7:31
🔄​	azure linux		1	4	4	313	1051	5:26
💚​	azure windows			4	4	316	1049	7:33
🟨​	gcp linux	3		1	4	313	1053	8:45
🟨​	gcp windows	3		1	4	315	1051	13:20

9 interesting tests: 4 SKIP, 3 KNOWN, 1 RECOVERED, 1 flaky

	Test Name	aws linux	aws windows	azure linux	azure windows	gcp linux	gcp windows
💚​	TestAccept	💚​R	💚​R	💚​R	💚​R	💚​R	💚​R
🙈​	TestAccept/bundle/invariant/no_drift	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_endpoints/drift/recreated_same_name	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_indexes/recreate/embedding_dimension	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/ssh/connection	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🔄​	TestClustersGet	✅​p	✅​p	🔄​f	✅​p	✅​p	✅​p
🟨​	TestFetchRepositoryInfoAPI_FromRepo	💚​R	💚​R	💚​R	💚​R	🟨​K	🟨​K
🟨​	TestFetchRepositoryInfoAPI_FromRepo/root	💚​R	💚​R	💚​R	💚​R	🟨​K	🟨​K
🟨​	TestFetchRepositoryInfoAPI_FromRepo/subdir	💚​R	💚​R	💚​R	💚​R	🟨​K	🟨​K

Top 6 slowest tests (at least 2 minutes):

duration	env	testname
8:43	gcp windows	TestAccept
6:25	azure windows	TestAccept
6:15	aws windows	TestAccept
3:01	aws linux	TestAccept
2:52	azure linux	TestAccept
2:51	gcp linux	TestAccept

[renaudhartert-db]

Pushed fixes for the two red checks on the first CI run:

Integration test — TestFsMkdirWhenFileExistsAtPath/uc-volumes (real, PR-caused): fs mkdir over an existing file must surface as fs.ErrExist. The Files API reports this as HTTP 409, which the SDK's apierr.Code maps to codes.Aborted (not codes.AlreadyExists), so my codes-based checks missed it. Switched all filer error mapping from apierr.Code(err) == codes.X to matching the HTTP status directly (via a httpStatus helper), which is what the pre-existing implementation did. This also hardens the not-found mappings, which had the same latent mismatch. Modelled the "409 on create-directory-over-a-file" behavior in libs/testserver (it previously created the directory unconditionally, which is why no acceptance/unit test caught this) and added a Mkdir unit test.

lint — deadcode: the deadcode check (separate from golangci-lint) flagged four unreachable exported functions in the vendored libs/tmp copy (WithPartSize, WithAccountID, WithProfileFile, WithProfile). Added libs/tmp/ to the exclusion list in tools/check_deadcode.py, consistent with keeping the vendored copy byte-for-byte with upstream.