fix(parquet_derive): support raw identifiers as column names

[cbmixx]

Which issue does this PR close?

• Closes parquet_derive: cannot read or write columns whose name is a Rust keyword (raw identifiers like r#type become column "r#type") #10112.

Rationale for this change

#[derive(ParquetRecordReader)] and #[derive(ParquetRecordWriter)] could not
handle a Parquet column whose name is a Rust keyword (e.g. type). The only way
to spell such a field in Rust is a raw identifier (r#type), but the derives
stringified the identifier including the r# prefix:

• The reader's column-index lookup used name_to_index.get(stringify!(#field_names)),
  and stringify!(r#type) yields "r#type", so reading failed with
  ParquetError::General("column name 'r#type' is not found in parquet file!").
• The writer's Field::parquet_type() used self.ident.to_string(), which keeps
  the r# prefix, so the written schema got a column literally named r#type.

This made it impossible to read or write Parquet columns whose names are Rust
keywords, e.g. files produced by other Parquet writers with a column named type.

What changes are included in this PR?

Unraw the identifier (via syn::ext::IdentExt::unraw, already available through
the existing syn dependency) wherever it is used as a column name, while keeping
the raw identifier for field access in the generated code:

• parquet_derive/src/lib.rs: the reader derive builds a parallel list of unrawed
  field-name strings for the name_to_index lookup and its error message.
• parquet_derive/src/parquet_field.rs: Field::parquet_type() uses
  self.ident.unraw().to_string() for the schema column name.

Are these changes tested?

Yes. Added a unit test (test_parquet_type_with_raw_identifier) and an
integration round-trip test (test_parquet_derive_raw_identifiers) covering a
struct with a raw-identifier field (r#type) alongside a normal field, asserting
the schema columns are named type/count. I verified both tests fail without
the fix (the writer emits a column named r#type) and pass with it.

Are there any user-facing changes?

Structs with raw-identifier fields now read and write columns named without the
r# prefix. This is a bug fix; there are no public API changes. Code that somehow
relied on the previous r#-prefixed column names would change behavior, but such
names could not be produced by any other Parquet writer.

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AI disclosure (per CONTRIBUTING.md): this change was developed with the
assistance of an AI coding tool. I reviewed every line, verified the fix against
the failing/passing tests described above, and own the change.