fix: data race on Arrow v12 shared timestamp types (prewarm GetToTimeFunc)

[TangoEnSkai]

Context

close #179

Arrow v12's TimestampType.GetToTimeFunc lazily caches the type's *time.Location (via GetZone, which writes t.loc with no synchronization — apache/arrow#38795, fixed only in Arrow versions this driver can't move to yet, see #228). NewArrowRowScanner calls GetToTimeFunc on the shared arrow.FixedWidthTypes.Timestamp_us singleton for every result set, so the first concurrent queries race on that call. In the issue thread, @7phs posted the init() prewarm workaround and @bombsimon asked for it to live in the driver instead of every consumer — this PR does exactly that.

What

• Add a package init() in internal/rows/arrowbased that calls GetToTimeFunc() once on all four shared fixed-width timestamp singletons (Timestamp_s/_ms/_us/_ns), so the location cache is written before any concurrency is possible and every later call is a plain read.
• Add TestSharedTimestampGetToTimeFuncConcurrency, which hits the shared singletons from 32 goroutines. Verified both ways locally: with the init() it passes under -race; with the init() renamed away it fails with race detected during execution of test.
• CHANGELOG entry under ## Unreleased.

Why

• Prewarming is the minimal fix that works while the driver stays on Arrow v12 (the v12 pin is deliberate for backward compatibility, per Revert update Apache Arrow to v16 #228). Once warmed, GetZone only ever reads t.loc, so the race is gone rather than hidden.
• Doing it in the driver removes the footgun @bombsimon described: consumers currently have to import Arrow — a transitive dependency — and replicate the init() in their own code just to use databricks-sql-go safely from multiple goroutines.
• All four singletons are warmed (not just Timestamp_us, the one NewArrowRowScanner uses) to match the workaround users already deploy and to keep any other shared-singleton use safe.

Completion Criteria

• go build ./..., go vet ./..., gofmt clean on touched files
• Full unit suite passes (go test ./...)
• New test passes under -race with the fix, fails under -race without it
• CI green

Note for reviewers: go test -race ./internal/rows/arrowbased/ also reports pre-existing, unrelated races in TestCloudFetchIterator_CloseReleasesInFlightDownloads / TestCloudFetchIterator_CloseReleasesAfterRetry (a container/list race in the CloudFetch iterator). Those fail identically on main without this change.