Forward pipeline context, add other_context to JoinTransform, drop SerializeAsAny

- NarwhalsPipelineModel.__call__ now forwards the caller's context to
  source(context) rather than passing NullContext, so context-keyed sources
  (e.g. TPCHDataGenerator with TPCHTableContext) can be used directly without
  per-table adapter wrappers.

- JoinTransform gains an other_context field so a single context-keyed source
  can be reused across multiple joins (e.g. one TPCHDataGenerator instance
  serving customer/orders/lineitem). Validator now checks
  isinstance(other_context, other.context_type) at construction.

- Drop SerializeAsAny from source and other fields. ccflow's BaseModel
  metaclass already wraps bare BaseModel-typed fields, so the explicit
  annotation was redundant. Verified subclass info still survives JSON
  round-trip in tests.

- Notebook polish: drop hardcoded TOC, drop graph-awareness subsection,
  drop trailing Pointers section, fix ascii alignment in section 7
  diagram, and remove the TPCHTableProvider adapter (and per-table
  providers) by leaning on context passthrough -- pipelines now use the
  generator directly.

- Add tests for context forwarding to source, other_context flow-through,
  and other_context type-mismatch rejection (36 tests total).

Signed-off-by: Pascal Tomecek <pascal.tomecek@cubistsystematic.com>

Author: ptomecek