Introduce ConfigurationError For Client Setup

[GefMar]

Goal

Replace indistinct ValueError mapping for raw client setup failures with a stable public configuration error type.

Context

create_raw_client currently catches _foghttp.FogHttpError and raises ValueError(str(exc)). That keeps construction failures user-visible, but it erases whether the failure came from TLS, proxy endpoint parsing, numeric config or runtime setup. Tests then match text strings, which is fragile for a stability milestone.

Scope

• Add a public ConfigurationError in the FogHTTP error hierarchy.
• Map raw client setup/configuration failures to ConfigurationError.
• Consider subclassing ValueError if needed to preserve compatibility with existing callers.
• Update tests to assert the error type and stable diagnostic fields/substrings rather than brittle full text.
• Document the setup/configuration error class where public errors are listed.

Out Of Scope

• Retyping every runtime request error.
• Changing successful client construction behavior.

Acceptance Criteria

• Client setup/config failures use ConfigurationError.
• Compatibility with existing ValueError handling is preserved or explicitly documented as a 0.4.0-breaking decision.
• Tests do not depend on fragile full error-message text.
• Public docs list the new error type.

Test And Check Strategy

• uv run pytest tests/client_proxy tests/client_tls tests/client_options tests/test_raw_client_errors.py
• uv run mypy foghttp
• uv run pre-commit run --all-files

Documentation Impact

• Update error docs and 0.4.0 release notes.

Risks

• Risk: changing exception type surprises early adopters.
  Mitigation: consider subclassing ValueError and document the migration path.