fix: clear stale socket references on neighbors in remove_component

[chuenchen309]

Problem

Pipeline.remove_component resets only the removed component's own sockets,
but leaves the removed component's name behind on its neighbors' sockets:

• a downstream component keeps it in its input socket's senders
• an upstream component keeps it in its output socket's receivers

graph.remove_node drops the edges from the graph, but the senders/receivers
lists live on the socket objects held by the surviving component instances, and
those are never touched.

These dangling references corrupt pipeline introspection and validation:

• Pipeline.inputs() computes is_mandatory = socket.is_mandatory and not socket.senders. A stale sender makes not socket.senders false, so a
  now-unconnected mandatory input is hidden from inputs().
• validate_input then raises a spurious "... is already connected to component '<removed>'" error — naming a component that no longer exists — when the user
  tries to feed that input directly.

Reproduction

from haystack.core.pipeline.base import PipelineBase
from haystack.testing.factory import component_class

Producer = component_class("Producer", output_types={"value": int})
Consumer = component_class("Consumer", input_types={"value": int})

p = PipelineBase()
p.add_component("producer", Producer())
p.add_component("consumer", Consumer())
p.connect("producer.value", "consumer.value")

p.remove_component("producer")

sock = p.get_component("consumer").__haystack_input__._sockets_dict["value"]
print(sock.senders)   # ['producer']  -> dangling reference to a deleted component
print(p.inputs())     # {}            -> mandatory 'consumer.value' input is hidden

Fix

Strip the removed component's name from its neighbors' sockets, walking the
incident edges (which still carry the from_socket/to_socket objects) before
remove_node.

Tests

Added two tests in test/core/pipeline/test_pipeline_base.py (pure Python, no
heavy deps) covering the downstream-neighbor senders and upstream-neighbor
receivers cases. Both fail before the change and pass after. The
downstream test also asserts inputs() re-exposes the mandatory input and
validate_input no longer raises.

$ python -m pytest test/core/pipeline/test_pipeline_base.py -k "stale_senders_on_downstream or stale_receivers_on_upstream"
2 passed

ruff check / ruff format --check pass on the changed files. Added a release
note under releasenotes/notes/.

---

Disclosure: I used AI assistance (Claude) to help locate this stale-state bug
and draft the tests. I reviewed the change, ran the suite, and take
responsibility for its correctness.

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/core/pipeline
base.py
Project Total

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