feat(arrow-cast): move map cast tests to integration suite

[codewarnab]

Which issue does this PR close?

Part of #11033.

Rationale for this change

The public-behavior cast tests in arrow-cast/src/cast/mod.rs are being moved into a crate-level integration test suite so individual cast families are easier to locate and maintain. This is the map-test slice of that work.

What changes are included in this PR?

• Adds the arrow-cast cast integration-test target.
• Adds arrow-cast/tests/cast/.
• Moves the four map cast tests from the inline cast/mod.rs test module to tests/cast/map.rs.

Are these changes tested?

• cargo test -p arrow-cast --test cast
• cargo fmt --all -- --check

A targeted Clippy run is blocked by existing arrow-data warnings under the locally installed Clippy version.

Are there any user-facing changes?

No. This is a test-organization-only change.

AI usage disclosure

OpenAI Codex assisted with the mechanical test extraction. I reviewed the resulting changes and verified the targeted tests locally.

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[sdf-jkl]

Thanks @codewarnab LGTM.

@Jefffrey please take a look at the proposed split and lmk if you agree with the direction.