docs: drop unresolvable Arg._validate cross-ref (strict mkdocs)

The bind-time constraint validator's docstring linked [`Arg._validate`][] (a private method with no doc page), which 'mkdocs build --strict' rejects as an unresolved cross-reference (the one failure in CI). Use a plain code span; likewise convert a stray :func: RST ref. No code change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rustyconover

vgi/arguments.py

@@ -1514,7 +1514,7 @@ def validate_const_arg_constraints(
     """Enforce const-argument value constraints at bind time.
 
     Const arguments are bind-time scalars, so validate them once (not per batch /
-    per group). Reuses [`Arg._validate`][], raising [`ArgumentValidationError`][]
+    per group). Reuses ``Arg._validate``, raising [`ArgumentValidationError`][]
     for any value that violates a declared ``choices``/``ge``/``le``/``gt``/``lt``/
     ``pattern`` constraint — so a bad value fails fast at bind instead of silently
     reaching ``compute()``/``update()``. Shared by the scalar and aggregate bind

vgi/scalar_function.py

@@ -651,8 +651,8 @@ def _validate_param_varargs_min(cls, input_schema: pa.Schema) -> None:
     def _validate_const_param_constraints(cls, arguments: Arguments) -> None:
         """Enforce ``ConstParam`` value constraints at bind (choices/ge/le/gt/lt/pattern).
 
-        Delegates to the shared :func:`vgi.arguments.validate_const_arg_constraints`
-        so the scalar and aggregate paths enforce identically — a discovered
+        Delegates to the shared ``validate_const_arg_constraints`` helper so the
+        scalar and aggregate paths enforce identically — a discovered
         constraint (surfaced via ``vgi_function_arguments()``) is binding for the
         modern API, matching the legacy ``Arg`` descriptor path.
         """