test(e2e): retry the dbconnect pip fallback through transient Windows file locks

[rugpanov]

Why

The run_dbconnect e2e test's should setup virtual environment step flakes on the Windows shard. When the extension's install path leaves databricks-connect missing, the test's own ensureVenvHasDbConnect fallback runs pip install databricks-connect. While pip overwrites the venv's existing numpy it can hit:

ERROR: Could not install packages due to an OSError: [WinError 32]
The process cannot access the file because it is being used by another process:
'...\.venv\Lib\site-packages\numpy\_core\_add_newdocs_scalars.py'

— a transient lock (a scanner or a still-running install holding the file open). The fallback swallowed that error and moved on, so databricks-connect stayed missing and the downstream "Databricks Connect:" tree-item check timed out with Setup confirmation failed. ensureVenvHasKernelDeps had the same single-shot-then-swallow shape.

What

• src/test/retry.ts — a pure retryOnTransientError helper plus an isTransientFileLockError predicate (matches WinError 32 "used by another process" and WinError 5 "Access is denied", searching .message / .stderr / .stdout since Node's execFile hides the OSError in stderr). It lives under src/test/ rather than src/test/e2e/ — tsconfig.json excludes the e2e folder from the unit build, so a colocated unit test only runs when the module sits outside it; the e2e spec imports it via an explicit .ts extension (as e2e specs already do for src/ modules).
• src/test/retry.test.ts — unit coverage: success without sleeping, retry-then-succeed, linear backoff, give-up-after-N, non-transient-rethrow, onRetry reporting, and the predicate's positive/negative cases.
• run_dbconnect.ucws.e2e.ts — both fallback pip installs now run through the helper (3 attempts, 5s linear backoff), logging each retry so a real Windows UX regression stays visible in nightly logs. The final swallow+log and the ground-truth tree-item assertion are unchanged.

Test-only: no production code, settings, persisted state, telemetry, or when-clause flags — zero user-facing impact and nothing changes for shards that never hit the lock.

Verification

• retry.test.ts: 9 passing (written red-first, then green).
• Full unit suite (test:unit): 818 passing, 0 failing (10 pre-existing pending).
• e2e file type-checks under src/test/e2e/tsconfig.json (the only error is a pre-existing assert {type: "json"} in wdio.conf.ts, untouched here).
• eslint + prettier -c clean on all touched files.

This pull request and its description were written by Isaac.

[rugpanov]

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[rugpanov]

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• PR number: 2120
• Commit SHA: 50f9fd97eab5c55571fa6ea5bc833ee6f26dfe49

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