fix: honor target sorted flag and preserve map field metadata in cast_map_to_map

[Smallfu666]

Which issue does this PR close?

Related to #5097. (Intentionally not Closes — see the scope note; the "entries null buffer" part of the issue is not representable through safe Arrow APIs.)

Rationale for this change

cast_map_to_map had two real bugs and one latent hazard:

• It used the source sorted flag for the result, so a cast to a map type with a different sorted flag returned the wrong type.
• It rebuilt the entries/key/value fields from scratch, dropping field metadata (and target nullability), so data_type() did not equal the requested target.
• Casting the key type of a sorted map can reorder keys (e.g. sorted Utf8 ["10","2"] → Int32 [10,2]), which would silently produce a Map(.., sorted=true) that is no longer sorted.

What changes are included in this PR?

• Rename-only fast path: when key/value types and sort order are unchanged (the common Parquet key_value → Spark entries relabel), delegate to arrow's cast, which relabels to the target fields and preserves their metadata with no value transformation.
• Recursive/hand-built path: when a child type changes, recurse with Comet's cast_array; when the sort order downgrades (true → false), hand-build with the target flag. try_new is used so a malformed target returns Err instead of panicking, and the result data_type() equals the requested target.
• Sorted-order safety: reject to_sorted && (!from_sorted || key-type changed) — a cast cannot manufacture or preserve an ordering across a key-type change.
• Field-count guard before indexing entries [0]/[1] — a malformed entries struct (0/1/3+ fields) returns Err, never panics.

Scope note: an entries struct carrying a non-None null buffer is not constructible through the inspected safe Arrow constructors (StructArray::new/try_new and MapArray normalize an all-valid buffer to None), so entries-level null-buffer "preservation" is not an observable property and is not claimed. Map-level null preservation is covered.

How are these changes tested?

16 unit tests in cast.rs: rename-only fast path (full target schema + unchanged values/offsets/map nulls); sorted equal / true→false / false→true(Err) / true→true key-type-change(Err) / value-only-cast; metadata + child casts; empty maps; mixed null/empty rows; sliced maps; key+value casts; malformed targets (0/1/3 entry fields, non-nullable field with null) returning Err without panic.