Align regular expression substitution contracts - #9676

Summary

• Share signed count binding for Pattern.sub and subn, matching the Py_ssize_t conversion domain.
• Preserve unused mixed-type literals and literal-only templates, and omit None results from replacement callbacks.
• Retain mutable subject exports while releasing byte borrows around callbacks, so in-place mutations affect later searches without allowing resize. Reuse search state and avoid constructing matches for literal replacements.

Uses the existing derived argument binder, template compiler, and search engine; no per-function FuncArgs parsing.

Reference: CPython 3.15.0 substitution implementation and Clinic binders.

Validation

• Bounded CPython 3.15 reference probes cover count conversion and overflow, callback results, empty matches, mixed literals, live mutable subjects, export lifetime, and callback match metadata.
• Independent source review and formatting/whitespace checks passed.
• Focused VM, host-env and stdlib check and strict Clippy with rustpython-stdlib/threading: passed. Normal repository commit hooks also passed.
• A separate default-feature strict Clippy attempt encountered the unchanged _ctypes Arc<BufferWrite> non-Send/Sync lint. The unused helper introduced by this refactor was removed, not suppressed.
• RustPython runtime, full workspace/C-API suites, and cross-platform execution were not run. No test, Lib, workflow, or marker edits.

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