Replace `POST /api/2.0/workspace-files/import-file/{path}?overwrite=…`
with the multipart variant of `POST /api/2.0/workspace/import` (via the
SDK's `Workspace.Upload` + `format=AUTO`). The legacy endpoint is being
deprecated and the new endpoint is the strategic replacement.

The multipart variant is required because the JSON body of /workspace/import
is server-capped at 10 MiB; multipart accepts the same sizes import-file did
(verified up to 250 MiB against a real workspace), so DAB users shipping
wheels/jars/large files keep working.

Error mapping uses SDK sentinels via errors.Is rather than raw status/error
code comparisons:

- ErrNotFound → noSuchDirectoryError (or mkdir+retry under
  CreateParentDirectories mode); also catches RESOURCE_DOES_NOT_EXIST.
- ErrResourceAlreadyExists → fileAlreadyExistsError (the new endpoint
  reliably sets error_code RESOURCE_ALREADY_EXISTS).
- ErrInvalidParameterValue + "Requested node type" message →
  fileAlreadyExistsError (existing object's node type doesn't match the
  upload — file vs notebook collision).
- ErrPermissionDenied → permissionError.

Apply the same sentinel-based pattern to Delete, ReadDir, and Stat for
404 detection, matching the existing usage in bundle/direct/util.go and
following AGENTS.md's rule against branching on err.Error() string content.

DIRECTORY_NOT_EMPTY in Delete keeps an explicit ErrorCode check since the
SDK has no sentinel for it.

Test plan:
- libs/filer/workspace_files_client_test.go covers the new error mapping.
- libs/testserver/handlers.go extended with a multipart handler at
  /workspace/import that surfaces 409s from the shared fake as 400 +
  RESOURCE_ALREADY_EXISTS to match real-workspace shape.
- acceptance/internal/prepare_server.go normalizes multipart bodies
  (sorted form fields, file parts recorded as {filename, size}) so
  request fixtures stay deterministic.
- ~70 acceptance fixtures regenerated for the new request shape.
- End-to-end verified against a real workspace for files, all notebook
  types (.py / .sql / .ipynb / .lvdash.json / .scala / .r), dashboards,
  and a 60 MB binary upload.
- Integration test TestFilerWorkspaceNotebook assertion updated to assert
  the path with extension (tc.name) — matches the absPath returned in
  fileAlreadyExistsError. Same change as #5106.

Co-authored-by: Isaac

…ted goldens

Add a focused acceptance test (acceptance/bundle/sync/upload-edge-cases) that
exercises the multipart upload pipeline with the inputs that differ in shape
between the legacy import-file and the new /workspace/import endpoints:

- A 12 MiB binary file (above the JSON-body 10 MiB cap that the multipart
  variant lifts).
- An empty file (multipart encodes an empty content part distinct from JSON's
  empty string).
- A python notebook (auto-detected as NOTEBOOK; testserver mirrors extension
  stripping).
- A .lvdash.json dashboard descriptor (real workspace assigns DASHBOARD;
  testserver records the upload-side request shape).
- A non-ASCII filename (héllo.txt — multipart encodes filenames with quoted
  string rules).
- A filename with a space.

Assertions inspect out.requests.txt and pin: the set of multipart_form.path
values, that every upload sets format=AUTO.

Also restore 11 golden out.requests.txt files that the previous regenerate
sweep accidentally deleted (templates/telemetry/*, run/inline-script/**,
run/scripts/**, resources/volumes/change-schema-name) — they were present on
main and silently disappeared during the rebase that brought their goldens
together with the removal patch.

Co-authored-by: Isaac

… writes)

The /workspace/import endpoint reports an existing path with two different
error codes depending on contention:

  - 400 RESOURCE_ALREADY_EXISTS: sequential conflict (overwrite=false on existing path)
    Message: "<path> already exists. Please pass overwrite=true to overwrite it."

  - 409 ALREADY_EXISTS: concurrent contention (e.g. multiple deploy locks racing)
    Message: "Node with name <path> already exists. Please pass overwrite=true to update it."

Verified by direct probe against aws-prod and aws-prod-ucws — sequential
conflicts produce the first shape, but a 5-goroutine race on the same path
produces the second shape with status 409 and error_code ALREADY_EXISTS.

The original PR's errors.Is(err, ErrResourceAlreadyExists) only catches the
first shape, leaving locker.Lock() returning the raw API error instead of
falling through to assertLockHeld() — which is what TestLock asserts. Add
errors.Is(err, ErrAlreadyExists) for the second shape; both inherit from
apierr.ErrResourceConflict but we use the specific sentinels rather than
the broader parent (ErrResourceConflict also covers ErrAborted, which is
a different concept).

Co-authored-by: Isaac

The os.Getenv check was added in this PR's first commit but is never set in
any CI config or test environment. The pre-existing !WorkspaceTmpDir guard
already covers the actual problem (yamlfmt build fails on DBR workspace
filesystems). The env var was an unused escape hatch.

Co-authored-by: Isaac

Investigation against a real workspace (aws-prod-ucws) and Confluence page
4867063896 ("Workspace Import Export Endpoints"):

- The 200 MiB cap (databricks.workspaceFilesystem.maxImportSizeBytes) gates the
  legacy /workspace-files/import-file endpoint (Projects service / Armeria),
  not the new /workspace/import multipart variant (Webapp). The multipart
  endpoint accepts at least 450 MB of regular-file content empirically.

- The legacy endpoint additionally enforces a 305s server-side request
  timeout, which becomes the binding constraint at typical client bandwidth
  for files in the ~400-500 MB range. There is no client-side knob to extend
  it (databricks-sdk-go's HTTPTimeout is an inactivity timer that resets on
  every body read; http.Client.Timeout is left at 0).

- Notebook content (payloads with the `# Databricks notebook source` header,
  detected via format=AUTO) hits a separate 10 MiB cap on the server
  (databricks.notebook.maxNotebookSizeBytes) — enforced on both endpoints,
  so the migration is not a regression on that axis.

Co-authored-by: Isaac