test: make SSE-B2 integration expectations deterministic

[ffumero2003]

Context

The four SSE/sync integration failures in #1184 report none for inherited encryption while the assertions expect SSE-B2/AES256.

Root cause

CLI v5 intentionally inherits the destination bucket default when encryption is omitted. The session-scoped persistent test bucket can predate B2’s newer SSE-B2 default, so it may still be configured for no encryption. The test setup did not establish the default its assertions require.

Changes

Ensure the test-owned persistent bucket has the SSE-B2/AES256 default before running integration tests. Only update it when necessary. Keep the upload, copy, and sync encryption assertions unchanged, so they still verify inheritance. Add the required changelog fragment.

Verification

• Four issue cases plus two other sync variants: 6 passed against the local current simulator with fake credentials.
• Legacy-default embedded simulator: test_sse_b2 and all test_sync_up variants: 5 passed.
• Full no-secret integration session on Python 3.13 across CLI v3/v4/v5: 4 passed per version.
• Python 3.13 unit and coverage sessions: passed (509/513/512 tests passed for v3/v4/v5; remaining cases skipped), 83% coverage.
• Ruff check and format check on the changed fixture, plus towncrier validation: clean.

Closes #1184