fix: populate path_params when importing OpenAPI specs

[WeiMengze666]

Description

When importing an OpenAPI spec, path parameters (e.g. {id} in
/example/{id}) were silently ignored:

• The URL was stored with OpenAPI {param} syntax instead of being
  converted to posting's :param syntax.
• path_params on the imported request was always empty, so path
  parameters could not be edited or substituted in the UI.
• Path-item level parameters (shared across all operations on a path)
  were not read at all — only operation-level parameters were processed.

This affected both inline parameters and $ref parameters, and both
OpenAPI 3.0.x and 3.1.x specs.

Changes

• Convert {param} → :param in the URL when building each
  RequestModel during import.
• Merge path-item level and operation level parameters per the OpenAPI
  spec: operation-level parameters override path-item parameters with
  the same (name, in) combination.
• Populate request.path_params from parameters with in: path.

Testing

Five new tests added to tests/test_open_api_import.py:

• test_import_inline_path_params — inline path params mixed with
  query params, OpenAPI 3.1
• test_import_ref_path_params — $ref path params, OpenAPI 3.1
  (the exact scenario from Can't use query parameters as References in path endpoints #292)
• test_import_path_params_openapi_30 — inline path params,
  OpenAPI 3.0.x
• test_import_path_item_level_params — path-item level params merged
  with operation-level params
• test_import_path_item_params_operation_overrides — operation-level
  param overrides path-item param with same (name, in)

Fixes #292

[MLengl]

Just to be consistent and transparent - this should also be applied/automated for QueryParams which are also hidden/ignored at the URL field.