Align OrderedDict update and serialization controls - #9682

Summary

• Reuse shared update/init argument binding and exact-dictionary snapshots before destination callbacks.
• Match keys/items/sequence fallback and bounded three-value pair unpacking without length-hint calls.
• Preserve source, pair and iterator lifetimes with explicit cleanup order.
• Obtain reduction state before items, retain custom empty state, and reuse normal iterator checks without advancing the original iterator.

Reference: CPython 3.15.0 OrderedDict update and serialization.

Validation

• Independent source review and bounded CPython 3.15 probes cover mapping fallback, pair limits, callback mutation, finalizer lifetimes, reduction state and iterator mutation.
• Formatting and whitespace checks 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.

Unrelated pop/move/clear internals and iterator error-state behavior are outside this change.

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