Support parameter names with +, with extra allowed characters

Support parameter names with +, with extra allowed characters.

Some OpenAPI documents use e.g. `/foo/{+bar}`, which means the `bar` variable has a larged allowed character set.

But the generator today rejects it because it looks for the parameter named exactly `+bar`, and that's not found.

Start in the generator's parameter lookup for OpenAPI path templates, reproducing the issue with `/foo/{+bar}` and a `bar` parameter. Trace why the `+` form is not matched; done means generation accepts the template and preserves the expanded character allowance.

Written by the indexing model from the issue text.