fix: harden RuntimeState serialization across entity fields

Adds missing serializers, discriminators, and exclude markers on entity
fields that previously crashed model_dump_json or restored ambiguously:

- Flow.persistence: add _serialize_persistence; drop | Any escape hatch
- Flow.input_provider: SerializableInstance dotted-path round-trip
- BaseAgent.agent_executor: add _serialize_executor_ref
- BaseAgent.tools_handler / cache_handler: exclude=True
- Memory / MemoryScope / MemorySlice: memory_kind Literal discriminator
- Knowledge.storage / .embedder: exclude live client, serialize spec
- BaseKnowledgeSource subclasses: source_type Literal + dict-resolver
- BaseKnowledgeSource.storage / chunk_embeddings: exclude=True
- input_provider: enforce InputProvider protocol via dedicated
  validator/serializer; reject non-class dotted paths in
  _dotted_path_to_instance
- MemoryScope/MemorySlice: allow restore without live Memory; expose
  bind() to reattach the dependency post-restore
- Knowledge.embedder: add BeforeValidator that resolves provider_class
  dotted paths back to a BaseEmbeddingsProvider subclass