Move numeric and boolean cast helpers into a private submodule

[anandghegde]

Which issue does this PR close?

Part of #11032 — the numeric and boolean conversion helpers group only, so it
does not close the issue.

Sibling PRs against the same issue: #11075 (temporal helpers). The two are
independent and touch disjoint parts of mod.rs, but both add a mod line and
an import, so whichever lands second will want a trivial rebase.

Rationale for this change

arrow-cast/src/cast/mod.rs is ~14k lines. #5125 moved list, decimal,
dictionary, string, map, run-array and union support into private submodules,
and #11032 lists the groups still inline. The numeric and boolean conversions
are one of them.

What changes are included in this PR?

A new arrow-cast/src/cast/numeric.rs holding:

• cast_numeric_arrays, try_numeric_cast, num_cast, numeric_cast
• cast_numeric_to_bool, numeric_to_bool_cast, cast_num_to_bool
• cast_bool_to_numeric, bool_to_numeric_cast, single_bool_to_numeric

cast_with_options stays the top-level dispatcher in mod.rs.

Worth flagging for review:

• The public API is unchanged. num_cast, cast_num_to_bool and
  single_bool_to_numeric are pub, so mod.rs re-exports the module with
  pub use crate::cast::numeric::* (the same shape already used for union).
  arrow_cast::cast::num_cast and the other two resolve exactly as before.
• Visibility is minimal rather than uniform. Only the three helpers the
  dispatcher actually calls — cast_numeric_arrays, cast_numeric_to_bool,
  cast_bool_to_numeric — became pub(crate). try_numeric_cast,
  numeric_cast, numeric_to_bool_cast and bool_to_numeric_cast have no
  callers outside the moved code and stay private to the new module.
• The group is not contiguous in mod.rs. cast_numeric_to_binary and
  adjust_timestamp_to_timezone sit between the numeric and boolean halves and
  belong to other groups, so they are left where they are.

Are these changes tested?

Covered by the existing arrow-cast tests — this is a code move, so no new
tests are warranted and no existing test was modified.

cargo test -p arrow-cast     379 passed + 12 passed, 0 failed (same as base)
cargo clippy -p arrow-cast --all-targets -- -D warnings   clean
cargo fmt -p arrow-cast -- --check                        clean
cargo build -p arrow                                      builds

Two checks beyond the suite, because a code move is only worth anything if it is
faithful and the public paths survive:

• Extracting the two original line ranges from HEAD and normalising away the
  added pub(crate) gives a byte-identical match against the new file.
• A temporary integration test calling arrow_cast::cast::num_cast,
  ::cast_num_to_bool and ::single_bool_to_numeric through their public paths
  compiles and passes. It is not part of this diff — it existed only to confirm
  the re-export, and was removed.

Run on macOS 26.6.2, aarch64-apple-darwin, toolchain 1.98.1 from
rust-toolchain.toml.

Are there any user-facing changes?

No. No public API change and no behavioural change.

AI usage disclosure

Written with AI assistance (Claude). The AI performed the mechanical extraction
and ran the verification above; I reviewed the result and own the change.

The moved bodies are unedited, and the byte-identity check is the evidence for
that rather than a reading. The judgement calls — which helpers need
pub(crate), which stay private, keeping the two interleaved functions from
other groups in place, and re-exporting to preserve the public paths — are
listed above because they are exactly the parts a "pure move" diff would not
otherwise draw attention to.