MCP Settings is a frozen dataclass with hand-rolled env parsing while the rest of the library is pydantic #143

Settings in the MCP server config is a frozen dataclass:
https://github.com/OpenSemanticLab/osw-python/blob/feat/mcp-server/src/osw/mcp/config.py#L45

Every other configuration-shaped object in osw is pydantic: OswBaseModel and all the *Param classes. Consequences of the outlier:

• max_results and max_chars are parsed by a hand-written _int_env helper with its own ValueError to RuntimeError translation, which pydantic does for free.
• Nothing validates domain (a bare domain vs a full URL is a real distinction here, see _derive_domain), sparql_endpoint, or state_dir.
• The type annotations are enforced by nothing at runtime, and src/osw/mcp is excluded from ty (pyproject.toml), so they are not enforced statically either.

Suggested fix: convert to a pydantic model, ideally BaseSettings-style with env aliases, replacing _first_env and _int_env. Natural point to do it is right after the config module moves to the shared service package in the MCP/CLI refactor, so it is one change instead of two.

Depends on: #133.