Infer CSV headers from sampled column types

[Jo2234]

Which issue does this PR close?

• Closes Option to Automatically Infer if CSV file has a header #6882.

Rationale for this change

Format currently requires callers to set with_header explicitly, even though schema inference already samples CSV records and applies Arrow's type inference rules. This makes the default easy to get wrong for the common case where a CSV begins with textual column names.

What changes are included in this PR?

• Add an opt-in Format::infer_format API that samples records and updates the format's header setting.
• Classify the first record as a header only when at least one first-row value is text while the sampled values in that column resolve to a non-text, non-null Arrow type. This follows the type-consistency signal used by DuckDB's CSV sniffer while deliberately leaving all-string inputs ambiguous.
• Reuse the existing CSV parser configuration and type inference, including delimiter, quote, escape, terminator, comment, null, and truncated-row settings.
• Keep ambiguous inputs conservative: empty, single-record, and all-string samples remain headerless.
• Document the API with a runnable example.

The supplied reader is consumed for inference, like infer_schema; callers can rewind or create a new reader before parsing.

Are these changes tested?

Yes. Added behavioral coverage for:

• headers with typed data columns;
• data without a header;
• empty, single-record, and all-string ambiguity;
• configured CSV delimiters; and
• bounded sampling via max_records.

Validation run locally:

• CARGO_BUILD_JOBS=2 cargo test -p arrow-csv --all-features — 77 unit tests and 13 doctests passed.
• CARGO_BUILD_JOBS=2 cargo test -p arrow --features csv — 157 unit/integration tests and 11 doctests passed.
• CARGO_BUILD_JOBS=2 cargo clippy -p arrow-csv --all-targets --all-features -- -D warnings — passed.
• CARGO_BUILD_JOBS=2 RUSTDOCFLAGS='-D warnings' cargo doc -p arrow-csv --no-deps — passed.
• cargo fmt --all -- --check — passed.
• git diff --check — passed.

Are there any user-facing changes?

Yes. This adds a non-breaking public API. Existing defaults and parsing behavior are unchanged unless a caller explicitly invokes infer_format.

AI assistance disclosure

AI assistance was used for implementation and test drafting. The resulting change was reviewed against the existing CSV parser and type-inference behavior, and the validation commands and results above were run on this branch.