Add unit tests for L2VPN state machine transitions

[gretelliz]

Description

This PR adds new unit tests to validate the current and desired behavior of the L2VPN connection state machine.

Important:
This PR does not modify any production code in sdx-controller (no changes to sdx_datamodel.connection_sm, connection_handler.py, or any other module).
It only introduces new test cases in the new file test_connection_state_machine.py to document and verify the state machine behavior.

These tests are intended to:

• Serve as living documentation of the expected transitions
• Help detect regressions if/when the state machine is updated
• Highlight gaps between current implementation and requirements (via xfailed tests)

This PR closes atlanticwave-sdx/datamodel#199
(and builds upon the context from atlanticwave-sdx/datamodel#184).

What the tests verify

The new tests cover the following specific requirements:

1. DOWN → MODIFYING (via MODIFY action)
2. DOWN → DELETED (via DELETE action)
3. ERROR → MODIFYING (via MODIFY action)
4. DISABLED → DELETED (via DELETE action)
5. MODIFYING → DELETED (via DELETE action)
6. Existence of the new MAINTENANCE state (test_maintenance_state_exists)
7. UP → MAINTENANCE (via MW_START action)
8. MAINTENANCE → UP (via MW_END action)

Current test results (as of this PR)

• 7 passed
• 2 xfailed (UP → MAINTENANCE and MAINTENANCE → UP)

Reason for the two xfailed tests:
The current connection_state_machine() implementation does not raise ConnectionSMException for these transitions.
The xfailed status serves as a marker that these transitions still need proper implementation / validation in a future PR.

Related issues

• Closes create a basic test for issue 184 on sdx controller datamodel#199
• Related to Review L2VPN state machine datamodel#184
• Triggered by observations in SDX-Controller fails to remove L2VPN in down state - return error 500 #461

[coveralls]

Pull Request Test Coverage Report for Build 23260118717

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 53.876%

---

Totals
Change from base Build 22150611686:	0.0%
Covered Lines:	1265
Relevant Lines:	2348

---

💛 - Coveralls

[YufengXin]

@gretelliz This looks good. But I am not sure about the two xfail's:
2 xfailed (UP → MAINTENANCE and MAINTENANCE → UP)
In datamodel, they're allowed and should succeed.

[YufengXin]

Do the two xfail cases related to the 'maintenence' pass?

[gretelliz]

Hi @YufengXin,

Thanks for reviewing the PR!
The two xfail tests (UP → MAINTENANCE and MAINTENANCE → UP) are still failing.
I left them as xfail on purpose because according to the datamodel those transitions should be allowed, but the current state machine implementation in the controller doesn't seem to be handling them correctly yet.
They serve as documentation of the gap we need to close later.