[Minor] Migrate src/plugins/lua/contextal.lua to lua_shape

[Minor] Migrate src/plugins/lua/contextal.lua to lua_shape

Replace tableshape with lua_shape in Contextal plugin.

Changes:
- Uses lua_redis.enrich_schema() which returns lua_shape with mixins
- ts.string, ts.boolean, ts.number → T.string(), T.boolean(), T.number()
- ts.array_of(x) → T.array(x)
- :is_optional() → :optional()
- Added comprehensive documentation to all fields

Schema: config_schema with Redis common fields plus Contextal-specific
settings for API URLs, caching, timeouts, and custom actions.