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 21653251912

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 53.895%

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Totals
Change from base Build 20488537231:	0.0%
Covered Lines:	1266
Relevant Lines:	2349

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💛 - Coveralls