docs: update e2e README with comprehensive test suite reference

[oboehmer]

Description

The tests/e2e/README.md was significantly outdated — it documented only 3 of 15 test classes, ~35 of 62 base tests, and 3 of 11 fixture directories. This PR rewrites it to accurately reflect the current state of the E2E test suite.

Closes

N/A

Related Issue(s)

N/A

Type of Change

• Documentation update

Test Framework Affected

• N/A (not test-framework specific)

Network as Code (NaC) Architecture Affected

• N/A (architecture-agnostic)

Platform Tested

N/A — documentation-only change.

Key Changes

• Updated test class hierarchy diagram (3 → 15 classes) with annotations (@windows, issue refs)
• Updated base class test count (~35 → 62) and documented all 13 test categories with accurate counts
• Added complete directory tree including all 11 fixture directories and full mocks/ structure
• Added scenarios reference table (14 scenarios with architecture, test counts, exit codes)
• Added CI/CD execution details from .github/workflows/test.yml (Linux + Windows matrix)
• Added test markers section (@pytest.mark.e2e, @pytest.mark.windows, skipif)
• Added fixtures reference (4 shared session-scoped + 16 E2E-specific)
• Added key module reference with one-liner descriptions
• Updated "Adding a New Scenario" guide with current E2EScenario dataclass fields

Testing Done

• Self-review of documentation accuracy against source code

Test Commands Used

# Documentation-only change — no test commands needed

Checklist

• Self-review of code completed
• Documentation updated (if applicable)
• No new warnings introduced

Additional Notes

All counts and details were verified against the actual source files (config.py, conftest.py, test_e2e_scenarios.py, html_helpers.py, .github/workflows/test.yml).