Fix _interpreters shared and updates argument diagnostics - #9295

Summary

• Use the Python 3.15 parameter names shared and updates instead of numeric argument labels in dictionary-validation errors.
• This covers _interpreters.exec, run_string, run_func and set___main___attrs through their existing argument converters.
• Extracts two native diagnostic literals from Integrate remaining Python 3.15 final runtime and libraries #8954; no Lib or test changes.

Validation

• Built the default+CAPI debug CLI from current main ad8bd18 plus only this patch.
• Passed changed-file hooks including rustfmt, VM checks with and without threading, threading test-target compilation, and scoped VM all-target strict Clippy (-D warnings).
• Ran only the unchanged CommonTests.test_signatures method from CPython 3.15 pin d220409a13b77de7a6c30abe16ea90db01528edc, preserving all five assertions. The external isolation sets id=0 and skips both lifecycle fixtures; the canonical module itself is not executed.
• Pinned CPython and patched RustPython pass all five assertions. Unpatched main fails at the first expected regex mismatch (argument 3 rather than argument 'shared'); the remaining four assertions are not reached on baseline.
• Verified native module origin, executable/source hashes and stdlib provenance. Runs use -B, a 120-second timeout, 1 GiB address-space limit and zero core dumps.

This validates early argument errors only, not subinterpreter lifecycle or execution capability. Full interpreter modules and the full workspace/CAPI runtime suite were not run.

Prepared and validated with AI assistance.