Share native typing constructor binding and validation - #9686

Summary

• Replace three duplicated constructor parsers with shared raw FromArgs/bind_for layouts and variance checks.
• Validate names and argument layout before invoking variance truth callbacks, preserving callback and error order.
• Normalize an explicit TypeVar bound=None and check non-None bounds before constraint count/conflict validation.
• Keep raw control objects alive through constructor metadata setup while preserving default, bound, name and evaluator identities.

Reference: CPython 3.15.0 native type parameter constructors and generated Clinic wrappers.

Validation

• Independent source review and bounded CPython 3.15 reference probes cover binding errors, non-string names, variance callbacks/finalizers, bound/constraint ordering, checked-bound None results and identity preservation.
• 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.
• Strict Clippy with the same packages/features and -- -D warnings passed. This is a focused threading-enabled subset of CI, not the full workspace/all-targets suite.
• RustPython runtime, full workspace/C-API suites and cross-platform execution were not run. No test, Lib, workflow or marker edits.

Existing caller metadata architecture is unchanged. Argument error wording uses the shared binder's conventions where those differ from CPython. This change preserves TypeVarTuple metadata from #9113 and does not expand into TypeAliasType.

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