Split cast tests into an arrow-cast/tests test suite

### Is your feature request related to a problem or challenge?

Follow-up to #5125. This issue captures the remaining test-organization work identified after the original module extractions.

The test module in `arrow-cast/src/cast/mod.rs` contains approximately 11,400 lines and more than 300 tests. Most cast implementations have been divided into functional submodules, but their public-behavior tests remain collected in one large inline module.

This accounts for most of the remaining size of `cast/mod.rs` and makes tests for individual cast families difficult to locate and maintain.

### Describe the solution you'd like

Move the public-behavior tests currently in `cast/mod.rs` into a crate-level test suite, following the organization used by Parquet:

```text
arrow-cast/
├── src/
├── tests/
│ └── cast/
│ ├── mod.rs
│ ├── binary.rs
│ ├── decimal.rs
│ ├── dictionary.rs
│ ├── dispatch.rs
│ ├── list.rs
│ ├── map.rs
│ ├── numeric.rs
│ ├── run_array.rs
│ ├── string.rs
│ ├── struct.rs
│ └── temporal.rs
└── Cargo.toml
```

Configure the suite as one test target:

```toml
[[test]]
name = "cast"
path = "./tests/cast/mod.rs"
```

A small first PR could establish the test target and move one coherent group, such as the map tests. Subsequent PRs can reference this issue while moving the remaining groups.

Suggested tracking:

- [ ] Establish `tests/cast/mod.rs` and the `cast` test target
- [ ] Move a small initial group
- [ ] List and list-view tests
- [ ] Map tests
- [ ] Dictionary tests
- [ ] Decimal tests
- [ ] String tests
- [ ] Binary and byte-view tests
- [ ] Numeric and boolean tests
- [ ] Temporal, duration, and interval tests
- [ ] Run-array tests
- [ ] Struct tests
- [ ] General dispatch, null, and `can_cast_types` tests
- [ ] Remove the large inline test module from `cast/mod.rs`

Tests that specifically exercise private implementation details can remain colocated with their source modules. This issue primarily concerns tests currently exercising the public cast API from `cast/mod.rs`.

### Additional context

Parquet uses explicit test targets under `parquet/tests/`, including nested test suites rooted at files such as `tests/arrow_reader/mod.rs`. This proposal applies the same organization to `arrow-cast`.

In Cargo terminology these are integration tests, but they remain part of the `arrow-cast` package, use its development dependencies, and test `arrow_cast` through its public API.

Together with the separate implementation-extraction follow-up, this issue supersedes the remaining work under #5125 and allows that umbrella issue to be closed.

This can remain a single tracking issue. Separate follow-ups are only necessary if large groups such as decimal or temporal tests need independent ownership or coordination.

### AI usage disclosure

This issue was drafted with assistance from OpenAI Codex. Codex was used to inspect the existing `arrow-cast` and Parquet test organization and help formulate the proposed test structure. No code changes were generated or made.