…ialized

The fallback added for agent snapshot serialization errors preserved the
original runtime failure, but it could also replace the entire
chat_generator or tool_invoker payload with an empty dict. That made the
saved snapshot impossible to resume even when only a runtime-only field
such as a streaming callback was non-serializable.

This change narrows the fallback behavior: Haystack now retries those
component inputs field-by-field and omits only the fields that cannot be
serialized, preserving resumable fields like messages, state, and tools.
A regression test covers resuming from a tool-invoker snapshot created
with a non-serializable runtime callback.

Constraint: Must preserve the original deepset-ai#11108 goal of not masking the real runtime error
Rejected: Keep saving `{}` and document snapshots as non-resumable | breaks the existing resume contract more than necessary
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If agent snapshot fallback behavior changes again, verify both error preservation and snapshot resumability
Tested: hatch -e test run pytest test/components/agents/test_agent_breakpoints.py -k 'resume_from_tool_invoker' -q
Tested: hatch -e test run pytest test/core/pipeline/test_breakpoint.py -k 'create_agent_snapshot' -q
Tested: hatch run fmt-check haystack/core/pipeline/breakpoint.py test/components/agents/test_agent_breakpoints.py
Not-tested: Full unit suite and integration suite
Related: deepset-ai#11126

…puts fail to serialize

Address the Copilot review on deepset-ai#11127: when every field of a
chat_generator or tool_invoker input fails to serialize,
_serialize_agent_component_inputs previously returned a bare `{}`. The
downstream `_deserialize_value_with_schema` rejects `{}` with
DeserializationError, which would silently re-introduce the exact
non-resumable snapshot behavior the fix was meant to prevent (e.g. when
resuming from a ToolBreakpoint where the sub-component's inputs are
not strictly required).

The helper now always returns a structurally valid
`{"serialization_schema", "serialized_data"}` pair. When all fields
are omitted the payload degrades to the schema-valid empty object
(`{"type": "object", "properties": {}}` + `{}`), which deserializes
back to `{}` without raising.

Existing unit tests in TestCreateAgentSnapshot are updated to assert
the new empty-but-valid payload shape, and a new test verifies that
the all-fields-fail payload round-trips through
`_deserialize_value_with_schema` without raising. The release note is
extended to describe the empty-payload edge case.

Constraint: Must preserve the original deepset-ai#11108 goal of not masking the real runtime error
Constraint: Must keep the narrower field-by-field fallback from the previous commit intact
Rejected: Keep returning bare `{}` and document snapshots as non-resumable in this edge case | regresses the snapshot resume contract in the very scenario the previous commit promised to preserve
Rejected: Fall back to a different marker payload (e.g. string sentinel) | breaks downstream deserializer's object/properties contract and would require changes outside the fallback helper
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If agent snapshot fallback behavior changes again, verify both DeserializationError is not raised on the empty-fields case and that the helper never returns a bare `{}`
Tested: hatch -e test run pytest test/core/pipeline/test_breakpoint.py -k 'create_agent_snapshot' -q
Tested: hatch -e test run pytest test/components/agents/test_agent_breakpoints.py -k 'non_serializable_runtime_callback' -q
Tested: hatch -e test run pytest test/components/agents/test_agent_breakpoints.py -k 'resume_from_tool_invoker and not new_breakpoint' -q
Tested: hatch run fmt-check haystack/core/pipeline/breakpoint.py test/core/pipeline/test_breakpoint.py test/components/agents/test_agent_breakpoints.py
Tested: hatch run test:types haystack/core/pipeline/breakpoint.py
Not-tested: Full unit suite and integration suite
Related: deepset-ai#11126