fix(a2a): correct IndentationError, add tdd_issue_9250 tags, fix CONTRIBUTORS

- Fix 2-space -> 4-space indentation on _handle_session_close in
  facade.py; this single error caused every CI gate to fail
  (lint, typecheck, unit_tests, integration_tests, e2e_tests, security)
- Add @tdd_issue @tdd_issue_9250 tags to the three session_id
  validation scenarios in a2a_facade_coverage.feature per the mandatory
  bug-fix TDD workflow requirement
- Fix CONTRIBUTORS.md entry: was PR #11053 / issue #9094, corrected to
  PR #11098 / issue #9250

ISSUES CLOSED: #9250

features/a2a_facade_coverage.feature

@@ -24,16 +24,19 @@ Feature: A2A local facade coverage — uncovered handler and edge-case paths
   # Session close — session_id validation guard (lines 321-324)
   # -------------------------------------------------------------------
 
+  @tdd_issue @tdd_issue_9250
   Scenario: Session close with empty session_id and no service raises ValueError error
     Given a facade-cov facade with no services
     When I dispatch facade-cov operation "session.close" with params {"session_id": ""}
     Then the facade-cov response status should be "error"
 
+  @tdd_issue @tdd_issue_9250
   Scenario: Session close with missing session_id key and no service raises ValueError error
     Given a facade-cov facade with no services
     When I dispatch facade-cov operation "session.close" with params {}
     Then the facade-cov response status should be "error"
 
+  @tdd_issue @tdd_issue_9250
   Scenario: Session close with empty session_id and wired service raises ValueError error
     Given a facade-cov facade with a mock SessionService
     When I dispatch facade-cov operation "session.close" with params {"session_id": ""}