chore: Implement manual test plan for SMTP mock server flakiness fix

- Added a new documentation file outlining the manual test plan to validate the SMTP mock server flakiness fix, ensuring improved mail test reliability without affecting production behavior.
- Updated the current specification document to reflect the focus on stabilizing flaky SMTP STARTTLS+AUTH unit tests, including detailed research findings and requirements for the implementation.
- Created a QA/Security validation report for the SMTP flaky test fix, confirming that changes are test-only, stable under repeated runs, and do not introduce new security risks.

docs/issues/manual_test_smtp_mock_flakiness_fix.md

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+---
+title: Manual Test Plan - SMTP Mock Server Flakiness Fix
+status: Open
+priority: High
+assignee: QA
+labels: testing, backend, reliability
+---
+
+# Test Objective
+Confirm the SMTP mock server flakiness fix improves mail test reliability without changing production mail behavior.
+
+# Scope
+- In scope: test reliability for SMTP mock server flows used by backend mail tests.
+- Out of scope: production SMTP sending behavior and user-facing mail features.
+
+# Prerequisites
+- Charon repository is up to date.
+- Backend test environment is available.
+- Ability to run backend tests repeatedly.
+
+# Manual Scenarios
+
+## 1) Target flaky test repeated run
+- [ ] Run `TestMailService_TestConnection_StartTLSSuccessWithAuth` repeatedly (at least 20 times).
+- [ ] Record pass/fail count and any intermittent errors.
+
+## 2) Mail service targeted subset run
+- [ ] Run mail service connection and send test subset once.
+- [ ] Confirm no new intermittent failures appear in related tests.
+
+## 3) Race-focused verification
+- [ ] Run targeted mail service tests with race detection enabled.
+- [ ] Confirm no race warnings or hangs occur.
+
+## 4) Cleanup/shutdown stability check
+- [ ] Repeat targeted runs and watch for stuck test processes or timeout behavior.
+- [ ] Confirm test execution exits cleanly each run.
+
+# Expected Results
+- Repeated target test runs complete with zero flaky failures.
+- Related mail service test subset remains stable.
+- No race detector findings for targeted scenarios.
+- No hangs during test cleanup/shutdown.
+
+# Regression Checks (No Production Impact)
+- [ ] Confirm only test reliability behavior changed; no production mail behavior changes are required.
+- [ ] Confirm no production endpoints, settings, or user-facing mail flows are affected.
+- [ ] Confirm standard backend test workflow still completes successfully after this fix.