Align percent formatting with Python 3.15 final - #9125

Summary

• Align str, bytes and bytearray percent formatting with CPython 3.15 final: parse and convert fields in order, preserve mapping lookup/callback precedence, and include argument context in errors.
• Match keyed conversion restrictions, star width/precision conversion, unsupported-character diagnostics, and qualified conversion type names.
• Preserve main's invalid-character-length-before-index behavior from Check percent-c character lengths before index conversion #9052 and existing precision/integer conversion behavior from Fix percent-format precision and integer conversion #8876.
• Use fallible output reservation for width padding and add six small callback-order cases to the existing string snippet.

Focused extraction from #8954 onto main 0f93210. Only two Rust files and an appended snippet function; no Lib edits, dependency changes, compiler/IR work, OS APIs or XML changes.

Validation

• Fresh narrow release build with threading, stdlib, stdio, importlib and host_env passed on Linux x86-64.
• Eleven exact canonical final-source cases passed, each in a fresh process with a 30-second timeout and 1 GiB address-space limit; zero skips or expected failures.
• Existing Check percent-c character lengths before index conversion #9052 character-subclass regression and the new callback-order function passed separately under the same limits.
• cargo clippy --locked -p rustpython-common -p rustpython-vm --all-targets -- -D warnings, normal repository hooks and git diff --check passed.
• Preserved binary SHA256: 80a9afe1c3a2fb17db0183e18e6428d24c85b7dfd1c1cd6e4075a9cf9275745b.

Source review: the exact diff and runners passed independent review. Canonical method/helper ASTs match CPython d220409a13b77de7a6c30abe16ea90db01528edc; the runner uses minimal harness classes and requires 64-bit sizes before allowing methods with unreachable 32-bit allocation branches. Existing #9052 snippet logic is unchanged.

Limits

No full workspace/C-API suites, complete Python formatting/string/bytes suites, 32-bit/Windows/WASM runtime, concurrency or huge-allocation probes. Fallible width reservation is not a general huge-precision safety claim. No performance or human manual validation is claimed.

AI assistance: OpenAI Codex prepared the extraction, source review and validation tooling. The exact model version is not exposed by this runtime. Publication follows the maintainer’s authorized split workflow.