Fix datetime constructor and method signature metadata - #9532

Summary

Address the 13 datetime inspect.signature differences in the actual Ubuntu report from #9530:

• Supply the existing public defaults for timedelta, time.isoformat, datetime.isoformat, datetime.fromtimestamp, and datetime.astimezone.
• Use the existing typed argument structs for date, time, and datetime constructor signatures. Keep raw pickle-state handling in slot_new, before normal argument binding.
• Match the positional-only parameter names of fromisoformat and __format__.
• Enable --signature output in the existing CI whats_left step, using the reviewed one-line diagnostic change from Align builtin signatures exposed by whats_left #9530. It prints results already computed by that step; imports, builds, and scans are unchanged.

The constructor refactor preserves normal binding, pickle positional/keyword behavior, subclass wrapping, and argument destruction before instance allocation. No handwritten signature override, shared binder/parser change, Lib edit, or test expectation change is included.

Evidence

The comparison is against CPython 3.15.0 Clinic declarations and native constructor/default handling. Like CPython, missing native arguments can retain a sentinel while exposing their documented zero, None, or 'auto' default.

The source report measures normalized inspect.signature, not byte-for-byte raw __text_signature__. All 13 entries have a corresponding source correction here; their disappearance still needs runtime confirmation on this head.

Validation

• Independent source review, configured pre-commit hooks, and git diff --check: passed.
• Local compilation, interpreter tests, Rust tests, and clippy: not run; local dependency cache/bootstrap limitations remain.
• Existing CI is the runtime validation gate, particularly test_datetime, existing datetime snippets, and the whats_left signature report. Draft pending that evidence.

AI assistance: OpenAI Codex performed investigation, implementation, and source review; runtime limitations are disclosed above.