fix(arrow/scalar): align timestamp timezone parsing

[fallintoplace]

Rationale for this change

MakeScalarParam, ParseScalar, timestamp builders, JSON decoding, and typed CSV parsing should use the same timestamp timezone rules. This also adds support for Arrow fixed-offset timezone strings.

What changes are included in this PR?

• Validate timestamp timezone metadata through TimestampType.GetZone.
• Accept UTC metadata case-insensitively, including forms such as Utc.
• Require the input to contain an offset exactly when the timestamp type has a non-empty timezone.
• Keep timezone-naive values offset-free.
• Preserve the instant represented by an explicit input offset.
• Keep numeric timestamp values as raw epoch values after validating the type timezone.
• Validate CSV timestamp metadata before reading rows, including null-only columns.
• Keep timezone offsets in CSV output for timezone-aware timestamps so CSV writer and reader round trips work.
• Preserve an explicitly supplied empty custom timestamp layout.

Are these changes tested?

• go test ./arrow ./arrow/scalar ./arrow/array ./arrow/compute -count=1
• go test ./arrow/csv -run Timestamp -count=1
• go test -race ./arrow ./arrow/scalar ./arrow/array ./arrow/compute ./arrow/csv -run Timestamp -count=1

Are there any user-facing changes?

Timestamp string parsing now rejects mismatched timezone presence:

• timezone-less type plus no offset: accepted
• timezone-less type plus an offset: rejected
• timezone-aware type plus an offset: accepted
• timezone-aware type without an offset: rejected

Explicit offsets still determine the represented instant, and mixed-case UTC metadata remains accepted. CSV output now includes the offset for timezone-aware timestamps.

[zeroshade]

@fallintoplace need to fix the failing tests