fix(datagen): preserve timestamp time zones

[xixiGoodLuck]

Summary

• Preserve the requested timezone metadata when lance-datagen generates timestamp arrays.
• Prevent rand_struct from panicking when a struct contains a timezone-aware timestamp field.

Root Cause

FnGen constructed the output with PrimitiveArray::from(values), which restores the primitive type's default timestamp metadata. For timezone-aware timestamps, that dropped the timezone and made the generated child array type disagree with the struct field type.

Changes

• Rebuild ArrayData with the generator's declared data type only when the generated type differs.
• Add regression coverage for both direct timestamp generation and timestamp fields generated through rand_struct.

Testing

• cargo fmt --all -- --check
• cargo test -p lance-datagen --lib (12 passed)
• cargo clippy -p lance-datagen --all-targets -- -D warnings
• cargo check --workspace --tests --benches

Related Issue

Fixes #7912

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: QUIET

Plan: Pro Plus

Run ID: 5425779e-f85c-4c43-a7ef-00f83070df59

📥 Commits

Reviewing files that changed from the base of the PR and between ddbc23c and d986100.

📒 Files selected for processing (1)

• rust/lance-datagen/src/generator.rs

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📝 Walkthrough

Walkthrough

Changes

Timestamp type preservation

Layer / File(s)	Summary
Typed array generation and validation rust/lance-datagen/src/generator.rs	FnGen::generate rebuilds arrays when constructed metadata differs from the declared type, preserving timestamp timezones. Tests cover direct timestamp generation and timezone-aware timestamps nested in structs.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: u70b3

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: preserving timestamp time zones in datagen.
Description check	✅ Passed	The description is directly related to the code changes and their regression coverage.
Linked Issues check	✅ Passed	The change preserves timezone metadata and covers the rand_struct panic scenario from #7912.
Out of Scope Changes check	✅ Passed	The changes stay focused on the timezone preservation bug fix and matching regression tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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