Align JSON encoder callback state with Python 3.15 - #9671

Summary

Align the native JSON encoder's callback-visible state with Python 3.15:

• Retain and expose the caller's marker dictionary. Use native dictionary operations for subclasses, keep markers on failure, and propagate callback-induced deletion errors.
• Iterate unsorted exact dictionaries live so callbacks can update subsequent values or entries.
• Preserve the exact list returned by a dictionary subclass's items(), sort it using the shared list implementation, and validate pairs as they are consumed.
• Convert encoder flags through the existing ArgIntoBool binding helper before constructor validation.

The change reuses the VM's mapping, list sorting, dictionary iteration and native dictionary helpers. It makes the existing native dictionary deletion helper available to the standard-library crate rather than duplicating its implementation.

Reference behavior is defined by CPython's encoder construction, default callback handling, and mapping iteration.

Validation

• Formatting and whitespace checks pass.
• Bounded Python 3.15 reference probes cover marker visibility and retention, reentrant calls, marker dictionary subclasses, callback-driven dictionary/list mutations, lazy pair validation, in-place tuple sorting, temporary-object lifetime and flag-conversion order.
• cargo check --locked --offline -j1 -p rustpython-stdlib --features rustpython-stdlib/threading passed.
• cargo clippy --locked --offline -j1 -p rustpython-vm -p rustpython-host_env -p rustpython-stdlib --features rustpython-stdlib/threading -- -D warnings passed.
• Native RustPython runtime tests, full workspace/CAPI suites and cross-platform tests were not run. Reference probes establish target semantics, not runtime validation.
• Production Rust files only; no tests or workflows changed.

AI assistance: OpenAI Codex assisted with source comparison, implementation and reference probes under maintainer direction.