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

[zeroshade]

Thanks for aligning timestamp parsing across the scalar, array, JSON, and CSV paths. The core timezone-presence rule and fixed-offset handling look consistent, and CI is green.

I found three regressions that need addressing before this lands:

1. Invalid timestamp metadata can make CSV converters append no value, causing panics in fixed-size lists and multi-column inferred readers.
2. CSV inference classifies zoned ISO-8601 values as timezone-less timestamps and then rejects them during conversion.
3. Zoned timestamp scalars no longer round-trip through their own String() output and ParseScalar.

I included minimal reproducers inline. The affected core packages and focused CSV timestamp tests otherwise pass locally.

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[zeroshade]

Blocking: returning a no-op converter after setting r.err violates the CSV reader's one-input/one-builder-value invariant.

I reproduced two new panics:

• fixed_size_list<timestamp[s, tz=not/a_timezone]> panics when constructing the array because the parent appends one list while this converter appends zero child values.
• NewInferringReader with an invalid timestamp supplied through WithColumnTypes can panic because this column has zero rows while another column has one.

A regular list similarly produces an empty list rather than a null. Also, because this validation is lazy, an invalid timestamp nested in an all-null list is never validated at all.

Please validate timestamp metadata recursively and ensure an error-path converter still appends a null value.

[zeroshade]

Blocking: this stricter parser is inconsistent with CSV type inference. tryParse still uses arrow.TimestampFromString, so 2024-01-01T00:00:00Z is inferred as timezone-less timestamp[s]; this call then rejects the same value because it contains an offset.

Reproducer:

r := csv.NewInferringReader(strings.NewReader("2024-01-01T00:00:00Z\n"))
r.Next()
// r.Err(): timestamp value ... for type timestamp[s] must not include a zone offset

This succeeds on the merge base. Inference should apply the same zone-presence rule so zoned input advances to the existing timestamp[*, UTC] inference rung.

[zeroshade]

Requiring an offset for zoned timestamp types makes the package's own scalar string representation non-parseable. scalar.Timestamp.String() still emits no offset:

typ := &arrow.TimestampType{Unit: arrow.Second, TimeZone: "UTC"}
s := scalar.NewTimestampScalar(0, typ)
scalar.ParseScalar(typ, s.String())
// must include a zone offset

This round-trip succeeds on the merge base. Please update timestamp scalar formatting to include the appropriate offset for zoned types, analogous to the array and CSV formatting changes in this PR.