fix(arrow/scalar): align timestamp timezone parsing

[fallintoplace]

Rationale for this change

MakeScalarParam and ParseScalar should handle timestamp timezone metadata consistently. The shared parsing path needs to support Arrow fixed-offset timezone strings without regressing the existing case-insensitive UTC behavior.

What changes are included in this PR?

• Reuse TimestampType.GetZone for non-UTC timestamp metadata.
• Keep UTC, utc, and mixed-case forms such as Utc on the existing timezone-neutral path.
• Require an explicit offset when a non-UTC timezone type is used, while preserving the instant represented by an offset in the value.
• Add coverage for invalid timezone metadata and timezone-less values for zoned types.

Are these changes tested?

• go test ./arrow/scalar

Are there any user-facing changes?

Timestamp scalar parsing now validates non-UTC timezone metadata and rejects timezone-less values for zoned types. Explicit offsets in the input determine the instant. Mixed-case UTC timezone metadata remains accepted.