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

features/tdd_plan_generation_validate_logic.feature

@@ -3,7 +3,7 @@ Feature: TDD — PlanGenerationGraph validation logic
   I want the PlanGenerationGraph._validate() logic to respect explicit LLM FAIL responses
   So that generated code is rejected when the LLM indicates failure, regardless of code length
 
-  @tdd_issue_10746
+  @tdd_issue @tdd_issue_10746
   Scenario: FAIL response is respected for code longer than 10 characters
     Given I have a langgraph PlanGenerationGraph instance
     When I execute the langgraph validate node with generated code:
@@ -12,7 +12,7 @@ Feature: TDD — PlanGenerationGraph validation logic
       """
     Then the langgraph validation status should be "FAIL"
 
-  @tdd_issue_10746
+  @tdd_issue @tdd_issue_10746
   Scenario: PASS response is accepted when no FAIL present
     Given I have a langgraph PlanGenerationGraph instance
     When I execute the langgraph validate node with generated code:
@@ -21,7 +21,7 @@ Feature: TDD — PlanGenerationGraph validation logic
       """
     Then the langgraph validation status should be "PASS"
 
-  @tdd_issue_10746
+  @tdd_issue @tdd_issue_10746
   Scenario: Mixed PASS and FAIL prefers FAIL
     Given I have a langgraph PlanGenerationGraph instance
     When I execute the langgraph validate node with generated code:
@@ -31,17 +31,17 @@ Feature: TDD — PlanGenerationGraph validation logic
       """
     Then the langgraph validation status should be "FAIL"
 
-  @tdd_issue_10746
-  Scenario: No explicit PASS or FAIL and code length > 10 should fail
+  @tdd_issue @tdd_issue_10746
+  Scenario: No explicit PASS or FAIL keyword should fail
     Given I have a langgraph PlanGenerationGraph instance
     When I execute the langgraph validate node with generated code:
       """
-      This validation response contains no PASS/FAIL keywords but the code body is long enough
-      to have previously passed due to the length guard.
+      The validation response contains only neutral observations about the code
+      structure and design choices with no verdict keyword present.
       """
     Then the langgraph validation status should be "FAIL"
 
-  @tdd_issue_10746
+  @tdd_issue @tdd_issue_10746
   Scenario: PASS keyword in mixed-case is accepted (case-insensitive)
     Given I have a langgraph PlanGenerationGraph instance
     When I execute the langgraph validate node with generated code:
@@ -49,4 +49,3 @@ Feature: TDD — PlanGenerationGraph validation logic
       pass: minimal checks passed, nothing critical found
       """
     Then the langgraph validation status should be "PASS"
-