Narrow match_arg overload to str-only when several_ok=False

[Copilot]

The first match_arg overload accepted str | Iterable[str] for arg when several_ok=False, but passing an iterable with several_ok=False raises ValueError at runtime. The type signature should reflect this constraint.

• Narrowed arg from str | Iterable[str] to str in the several_ok: Literal[False] overload
• Added # type: ignore[call-overload] to tests that intentionally exercise the invalid-type error path

# Before: type checker allows this, but it raises ValueError at runtime
match_arg(["a", "b"], choices, several_ok=False)

# After: type checker correctly rejects it
match_arg(["a", "b"], choices, several_ok=False)  # error: No overloads match

# Valid usages unchanged
match_arg("ban", choices)                          # -> str
match_arg("ban", choices, several_ok=True)         # -> list[str]
match_arg(["a", "b"], choices, several_ok=True)    # -> list[str]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>match_arg should be overloaded based on several_ok</issue_title>
<issue_description>e.g.

https://github.com/rstudio/pins-python/blob/bdb58e1cba26bae899c73c19871366b417edf113/pins/_adaptors.py#L145-L149</issue_description>

Comments on the Issue (you are @copilot in this section)

@nathanjmcdougall Here's that similar example:

@overload
def create_adaptor(obj: DataFrame) -> DFAdaptor: ...
@overload
def create_adaptor(obj: Any) -> Adaptor: ...
def create_adaptor(obj: Any | DataFrame) -> Adaptor | DFAdaptor:
    if isinstance(obj, AbstractPandasFrame):
        return PandasAdaptor(obj)
    elif isinstance(obj, Adaptor):
        return obj
    else:
        return Adaptor(obj)
```</body></comment_new>
</comments>

• Fixes match_arg should be overloaded based on several_ok #31

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[t-t-sonarqube]

Quality Gate passed

Issues

• 0 New Issues
• 0 Fixed Issues
• 0 Accepted Issues

Measures

• 0 Security Hotspots
• 100.00% Coverage (92.50% Estimated after merge)
• 0.00% Duplicated Code (0.00% Estimated after merge)

Project ID: tonkintaylor_ruru_9df383a8-3ce9-4983-bccc-45fb680571e3

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