Extract shared AsyncClientBase to eliminate cross-module duplication

[btoron]

Is your feature request related to a problem? Please describe.

Error handling code (_parse_error_response(), _handle_http_error()) and base properties (baseUrl, headers, config) are duplicated identically across 5 async modules:

• ofsc/async_client/core/_base.py (~98 lines)
• ofsc/async_client/metadata.py (~91 lines)
• ofsc/async_client/capacity.py (~69 lines)
• ofsc/async_client/statistics.py (~69 lines)
• ofsc/async_client/oauth.py (~48 lines)

Additionally, Protocol type stubs are duplicated in core/resources.py and core/users.py.

Total duplicated code: ~400+ lines across modules. Any bug fix to error handling requires updating 5 locations.

Note: Issue #116 covers duplication within metadata.py using helper methods. This issue addresses duplication across all async modules.

Describe the solution you'd like

Create a shared AsyncClientBase class in ofsc/async_client/_base.py:

class AsyncClientBase:
    """Base class for all async API modules."""

    def __init__(self, config: OFSConfig, client: httpx.AsyncClient):
        self._config = config
        self._client = client

    @property
    def config(self) -> OFSConfig:
        return self._config

    @property
    def baseUrl(self) -> str:
        if self._config.baseURL is None:
            raise ValueError("Base URL is not configured")
        return self._config.baseURL

    @property
    def headers(self) -> dict:
        # Shared auth headers logic
        ...

    def _parse_error_response(self, response: httpx.Response) -> dict:
        # Single implementation
        ...

    def _handle_http_error(self, e: httpx.HTTPStatusError, context: str = "") -> None:
        # Single implementation with full exception mapping
        ...

All modules inherit from it:

• AsyncOFSMetadata(AsyncClientBase)
• AsyncOFSCapacity(AsyncClientBase)
• AsyncOFSStatistics(AsyncClientBase)
• AsyncOFSOauth2(AsyncClientBase)
• Core base class inherits too

Also create a shared Protocol in _protocols.py to replace duplicated protocols in core/resources.py and core/users.py.

Describe alternatives you've considered

• Mixin approach (less clean but doesn't require changing inheritance)
• Keep duplication but add tests to verify consistency (doesn't solve the root cause)

Additional context

This is a prerequisite for maintainable growth. As more modules are added (Statistics, Collaboration, Parts Catalog), each would currently copy-paste another ~100 lines of boilerplate. With a base class, new modules start at ~0 lines of boilerplate.

Related: #116 (metadata-specific refactoring)