fix: harden sdk public contract verification

[fas89]

Summary

Harden forge-cli-sdk verification around its public contract instead of relying on import/build smoke checks alone.

This PR adds direct tests for:

• top-level public exports and version stability
• fluid_provider_sdk.testing exports and bundled fixtures
• action/type/helper behavior (ProviderAction, ApplyResult, ProviderCapabilities, ProviderMetadata, hooks)
• ContractHelper parsing across the bundled sample contracts
• a minimal provider passing the bundled ProviderTestHarness

It also includes the two smallest runtime fixes the new tests exposed:

• ProviderCapabilities is now a real Mapping[str, bool]
• BaseProvider.get_provider_info() now returns a deterministic non-empty default description

Related PRs / Tracker

• Trello: https://trello.com/c/p44HjEwb/36-p3-forge-cli-sdk-verification-hardening
• No separate GitHub issue exists for this SDK-only card.

Verification

• python3 -m pytest -q
• python3 -m ruff check src tests
• python3 -m black --check src tests
• python3 -m build --sdist --wheel
• Verified built wheel and sdist include fluid_provider_sdk/py.typed
• Verified built wheel and sdist include fluid_provider_sdk/testing
• Installed the built wheel into a clean venv and imported both fluid_provider_sdk and fluid_provider_sdk.testing