MCP servers (e.g. Zapier) return JSON Schema union types like
`"type": ["string", "null"]` for nullable parameters. The existing
`validate_params()` and `cast_params()` methods expected only simple
strings as `type`, causing `TypeError: unhashable type: 'list'` on
every MCP tool call with nullable parameters.
Add `_resolve_type()` helper that extracts the first non-null type
from union types, and use it in `_cast_value()` and `_validate()`.
Also handle `None` values correctly when the schema declares a
nullable type.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
- test_channel_plugins: fix assertion logic for discoverable channels
- test_filesystem_tools: normalize path separators for Windows
- test_tool_validation: use python to generate output, avoid cmd line limits
