feat(instrument): Port LangGraph framework adapter (M2)

[mmercuri]

Summary

M2 fan-out PR completing the LangGraph framework adapter port. The
mechanical multi-file copy from ateam/stratix/sdk/python/adapters/langgraph/
(7 files, ~2,248 LOC) landed via the M1.C orchestration PR (#96, this PR's
base); this PR layers the M2 deliverables on top:

• Round-2 deliberation item 23 — STRATIX* deprecation alias.
  STRATIXLangGraphAdapter is now resolved through PEP 562
  module-level __getattr__. Accessing the old name still returns
  LayerLensLangGraphAdapter but raises a DeprecationWarning
  pointing at the v2.0 removal, fired only at attribute-access time
  so bare import keeps working without warnings.
• Round-2 deliberation item 20 — Pydantic v2-only manifest field.
  get_adapter_info() now sets requires_pydantic = PydanticCompat.V2_ONLY
  explicitly. Both the manifest wrapper (info()) and the subclass
  override now agree, so the atlas-app catalog UI shows the right
  compat badge regardless of which entrypoint the manifest emitter
  ends up calling.
• Dedicated unit-test module at
  tests/instrument/adapters/frameworks/test_langgraph.py — 29 tests
  modelled after the SmolAgents / Langfuse adapter test pattern.
  Mocks LangGraph at the SDK shape level (_FakeCompiledGraph with
  invoke / ainvoke); covers lifecycle, info() reporting,
  alias deprecation, graph wrapping (success + error), handoff
  detection, trace_node / trace_langgraph_tool decorators,
  wrap_llm_for_langgraph (sync + async), state adapter, and
  replay serialization.
• Expanded sample. samples/instrument/langgraph/main.py now
  builds a 3-node planner -> researcher -> writer graph instead of
  the single-node smoke. LLM call inside the writer uses a MockLLM
  (no API key required); HttpEventSink import is best-effort so
  the sample runs cleanly even before the M1.E transport PR (instrument: HTTP/OTLP transport sinks + extras matrix CI #98)
  lands. Added samples/instrument/langgraph/README.md covering
  topology, expected output, and a "going to production" path.
• Docs. docs/adapters/frameworks-langgraph.md clarified — the
  backward-compat section now mentions the DeprecationWarning and
  a new Pydantic-compatibility section explains the v2-only stance.

Reference template followed: the already-shipped LangChain adapter at
src/layerlens/instrument/adapters/frameworks/langchain/.

Pydantic compatibility

LangGraph >= 0.2 inherits langchain-core's Pydantic v2-only
requirement. The package's __init__.py calls requires_pydantic(V2_ONLY)
at import time and the manifest declares it on every entry point, so users
on v1 see a clear RuntimeError instead of an opaque ImportError mid-
callback.

Stacks on

• feat/instrument-frameworks-orchestration (PR instrument: orchestration framework adapters (M1.C part 1) #96, M1.C part 1) — provides
  the actual adapter package; this PR extends it.
• Indirectly on feat/instrument-base-foundation (PR instrument: base foundation (M1.A port) #93, M1.A) via instrument: orchestration framework adapters (M1.C part 1) #96.

Test plan

• uv run pytest tests/instrument/adapters/frameworks/test_langgraph.py -x
  --> 29 passed
• uv run mypy --strict src/layerlens/instrument/adapters/frameworks/langgraph
  --> Success: no issues found in 7 source files
• uv run ruff check src/.../langgraph tests/.../test_langgraph.py
  --> All checks passed
• test_lazy_imports.py (the two non-protocols-dependent guards)
  and test_default_install.py still green
• python samples/instrument/langgraph/main.py exits 0 with the
  3-node graph result + event-type counts printed
• Reviewer (m-peko) verifies the PEP 562 alias pattern + the
  requires_pydantic propagation matches the conventions the
  remaining M2 fan-out PRs will adopt.