fix: handle PEP 649 NameError in inspect.signature on Python 3.14

[starfolkai]

Summary

Fixes #263. On Python 3.14 (PEP 649), inspect.signature(fn) defaults to VALUE format and eagerly evaluates annotations. Functions with TYPE_CHECKING-only imports in their annotations raise NameError, the bare except blocks in framework.py dropped parameter introspection, and tasks taking a hooks argument crashed with TypeError: missing 1 required positional argument: 'hooks'.

• Adds a shared get_signature(fn) helper in util.py that uses annotationlib.Format.FORWARDREF on Python 3.14+ (and vanilla inspect.signature on 3.10–3.13). FORWARDREF wraps unresolvable names as ForwardRef objects without raising; we only inspect parameter names/kinds, so annotation values are never needed.
• Applies the helper in framework._call_user_fn_args and the hooks-detection signature call in _run_evaluator_internal_impl.
• Applies the helper in the @traced decorator in logger.py, which had the same latent issue (degraded I/O tracing rather than a crash, but same root cause).
• Adds a 3.14-only regression test that defines a task whose hooks annotation references a TYPE_CHECKING-only import.

Test plan

• New test_hooks_with_type_checking_only_annotation fails on the unfixed code (saw_hooks == []) and passes after the fix.
• nox -s test_core — 443 passed, 56 skipped, 12 xfailed.
• pytest src/braintrust/test_framework.py src/braintrust/test_logger.py — 162 passed, 1 skipped (the new <3.14 guard).
• Real Eval against the Braintrust API on Python 3.14 with the issue's repro snippet: 0 errors after fix vs 1 error (TypeError) before, hooks correctly injected.

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