* fix: resolve OpenAPI 3.x server variables in _create_default_client
When an OpenAPI spec defines server variables (e.g. `https://{region}.api.example.com/v1`),
the default values are now substituted before constructing the httpx client base URL.
Previously, the URL was used as-is, causing all requests to fail for specs that use
server variable templating.
Fixes#1681
* fix: use str.replace instead of format_map for server variable substitution
format_map applies Python string formatting rules, so variable names
like {api.version} would be treated as attribute access and raise errors.
Literal token replacement handles all valid OpenAPI variable names safely.
99eaeb8af4