fix(agents/graphs): unblock TDD validate tests + read-only _should_retry + max_context_files

- features/steps/tdd_plan_generation_validate_steps.py: drop duplicate
  @given/@then registrations (live in plan_generation_langgraph_coverage_
  steps.py; redefinition caused AmbiguousStep errors crashing all 8
  behave-parallel workers); have the @when step feed the docstring as the
  FakeListLLM response so each scenario tests _validate()'s parsing of a
  specific PASS/FAIL signal rather than the input code.
- features/tdd_plan_generation_validate_logic.feature: add the required
  @tdd_issue tag alongside @tdd_issue_10746 (enforced by
  features/environment.py); tighten scenario 4 wording to remove the
  reference to the obsolete length guard.
- robot/plan_generation_graph.robot: drop assertion for handle_retry node
  (retry is a conditional edge, not a fifth node) and update node-count
  check from 5 to 4; update Should Retry test to assert _should_retry does
  NOT mutate state (read-only contract for LangGraph conditional edges).
- src/cleveragents/agents/graphs/plan_generation.py:
  * _validate: persist retry_count increment in return dict (FAIL path and
    exception path) so LangGraph propagates it through the state graph.
  * _should_retry: remove state mutation (conditional-edge functions are
    read-only in LangGraph; mutations were silently dropped, causing
    retry_count to remain 0 forever and the graph to loop infinitely).
    Adjust comparison to retry_count <= max_retries because _validate has
    already incremented before _should_retry runs.
  * __init__: add max_context_files parameter (default 5, validated > 0)
    and wire it into _format_context_summary in place of the hardcoded 5,
    implementing the configurable-limits contract tested by
    features/agent_configurable_limits.feature.

ISSUES CLOSED: #10746

robot/plan_generation_graph.robot

@@ -83,10 +83,9 @@ Plan Generation Graph Builds Workflow With Correct Nodes
     ...    assert 'analyze_requirements' in nodes
     ...    assert 'generate_plan' in nodes
     ...    assert 'validate' in nodes
-    ...    assert 'handle_retry' in nodes
     ...    print(f'Graph has {len(nodes)} nodes')
     ${result}=    Run Process    ${PYTHON}    -c    ${script}    shell=True
-    Should Contain    ${result.stdout}    Graph has 5 nodes
+    Should Contain    ${result.stdout}    Graph has 4 nodes
     Should Be Equal As Integers    ${result.rc}    0
 
 LangGraph Graphs Package Exports Workflow Classes