Align CSV dialect binding and validation with CPython 3.15 - #9684

Summary

• Replace the bespoke format-options parser with shared FromArgs binding for all nine Dialect options.
• Keep reader, writer and register_dialect limited to their own positional inputs, then bind format parameters after their input handling.
• Match dialect character/lineterminator validation order and diagnostics, preserving missing-versus-None behavior and conversion lifetimes.

Reference: CPython 3.15.0 CSV dialect construction and validation.

Validation

• Independent source review and bounded CPython 3.15 reference probes cover all nine positional options, callback/error precedence, quotechar=None defaults and validation collisions.
• Independently reconstructed the current-main merge and the single import-block resolution. Main’s surrogate-safe registry and writer-operation changes are preserved; 68 additional combined CPython reference assertions passed.
• Formatting and whitespace checks passed.
• Normal commit hooks passed.
• Focused cargo check --locked --offline -j1 -p rustpython-host_env -p rustpython-vm -p rustpython-stdlib --features rustpython-stdlib/threading passed.
• The final current-main composition passed normal all-targets Clippy for the same focused packages/features. Four diagnostic headings and locations match the unchanged-main baseline: one POSIX needless-collect and three ElementTree lints. Strict -D warnings is no longer clean on this newer base; no warning suppression or unrelated baseline edits.
• RustPython runtime, full workspace/C-API suites and cross-platform execution were not run. No test, Lib, workflow or marker edits.

This change does not address existing dialect identity or the parser/writer scalar-string limitation for lone surrogates. Shared binder errors may name Dialect where CPython uses an anonymous parser; no bespoke error formatter is added. The branch now includes main through 9c77b331a9e4, including #9636 and #9639. The import conflict retains both the shared binder types and PyStrRef; the cumulative diff from main remains confined to CSV (+83/-110). Related #9635 is not included.

AI assistance: OpenAI Codex performed source comparison, implementation, reference probes and independent source review; model version was not exposed.