Raise Phase 6 test coverage with JWT, HTTP lifecycle, and CI matrix.

[aviabhijit55-ship-it]

Summary
Closes the Phase 6 testing gap from dev-plan/PHASE-6-testing-coverage.md — raise coverage beyond the thin per-phase suites and add cross-cutting / integration tests before v1.0.0.
Adds dedicated coverage for DI edge cases (circular and missing deps, singletons), pipeline order (guards → middleware → pipes → interceptors → handler → filters, plus short-circuit and filter catch), MCP error shape, JWT/config/guards, nested modules, HTTP tools/call lifecycle, events, logger STDIO safety, CLI install, and the NitroTestingModule harness.
Wires measurement and the 3.10–3.12 matrix: pytest --cov=nitrostack, pytest.ini, tox.ini, and CI (.github/workflows/test.yml) now run coverage on Python 3.10, 3.11, and 3.12.
Small production fixes required so those tests are real, not mocked: circular-dep detection, OAuth token/audience errors, scope helpers, EventEmitter on/off/once, and pipeline closure binding.
Plan DoD: 34 test_*.py files (target was 25+). Branch: phase-6-testing-coverage.

Test plan:

• 1. pytest tests --cov=nitrostack --cov-report=term-missing is green and line coverage is 50%+
• 2. find tests -name 'test_*.py' | wc -l is 25+
• 3. tox (or the CI matrix) passes on 3.10, 3.11, 3.12
• 4. Run the full suite twice and confirm the same pass/fail (no flakes)
• 5. Spot-check new files: test_di_edge_cases.py, test_pipeline_order.py, test_error_handling.py, test_lifecycle_http.py, test_transport_http.py
• 6. Confirm existing Phase 1–5 tests (test_tasks, test_oauth, widgets, CLI, transports) still pass
• 7. Confirm no raw traceback leaks on a failing HTTP/STDIO tool call (MCP error shape)