[Minor] Migrate lua_mime.lua from tableshape to lua_shape

[Minor] Migrate lua_mime.lua from tableshape to lua_shape

Replace tableshape with lua_shape in message_to_ucl_schema function:
- Convert ts.shape to T.table
- Convert :describe() to :doc({ summary = ... })
- Convert :is_optional() to :optional()
- Convert ts.array_of to T.array
- Convert ts.pattern to T.string({ pattern = ... })
- Convert ts.one_of to T.enum for simple value lists
- Convert ts.integer/string/boolean to T.integer()/string()/boolean()

Transforms nested schemas: headers_schema, part_schema, email_addr_schema,
envelope_schema. No functional changes, luacheck passes.