fix(bundles): tombstone the broken legacy ZepChatMemory build method

[erichare]

Bundle Separation Phase A — follow-up: tombstone the broken legacy Zep component

ZepChatMemory.build_message_history (the component's only action method) lazily imports the zep-python v1 API — ZepClient + zep_python.langchain.ZepChatMessageHistory. Both symbols were removed in zep-python 2.x, and the bundle's zep extra pins zep-python==2.0.2, so the method has been broken at runtime for as long as the pin has existed — and its ImportError guard misleadingly told users to pip install zep-python, which is already installed. (This predates the bundle move: identical code ships at src/lfx/src/lfx/components/zep/zep.py on release-1.11.0.)

The component is already legacy=True with replacement=["helpers.Memory"], so rather than hand-write a new integration against the 2.x SDK (the zep_python.langchain module no longer exists, and langchain-community's classic Zep history class also requires the removed v1 client), this tombstones it following the PythonCodeStructuredTool non-functional-stub precedent:

• build_message_history now raises a clear RuntimeError pointing at the Message History component (the designated replacement) instead of the misleading install hint. Deliberately not an ImportError, so nothing can misread it as a missing-dependency condition.
• Flow identity preserved: class/component name, display_name, description, inputs, the memory output wiring, and the -> Memory annotation (drives frontend output typing) are byte-identical — saved flows keep loading, i18n locale keys are unchanged, and migration_table.json needs no edits (append-only contract).
• Deterministic failure mode: the stub imports no zep_python at all, so behavior doesn't depend on which SDK version happens to be installed.
• New bundle tests pin the stub contract (src/bundles/lfx-bundles/tests/test_zep_component.py): frontend identity + output wiring, the actionable error (matches "no longer functions", names Message History, asserts no pip install hint and not an ImportError), and an AST check that no zep_python import sneaks back in.

3 new tests pass; the 117 shim/loader contract tests pass in the isolated lfx env; ruff clean. The now-vestigial zep = ["zep-python==2.0.2"] extra is left in place — removing a published extra is a metadata-breaking change; it can be emptied (like glean/tavily) via consolidate_bundles.py in a follow-up.

Base bundles/review-fixes (#13579) — stacks on top of the Phase A chain.

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Frontend Unit Test Coverage Report

Coverage Summary

Lines	Statements	Branches	Functions
	43.28% (57619/133123)	69.21% (7828/11310)	41.49% (1291/3111)

Unit Test Results

Tests	Skipped	Failures	Errors	Time
4940	0 💤	0 ❌	0 🔥	11m 26s ⏱️

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                   Coverage Diff                   @@
##             bundles/review-fixes   #13580   +/-   ##
=======================================================
  Coverage                        ?   58.34%           
=======================================================
  Files                           ?     2290           
  Lines                           ?   219837           
  Branches                        ?    34181           
=======================================================
  Hits                            ?   128260           
  Misses                          ?    90119           
  Partials                        ?     1458           

Flag	Coverage Δ
frontend	57.61% <ø> (?)

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