Phase 3d: Add comprehensive SubscriptionController test coverage

[devin-ai-integration]

Summary

Adds 15 new tests to SubscriptionControllerTest (19 → 34 total, 103 tests overall) to close behavioral coverage gaps identified by comparing the migrated Spring controller against the original Struts EditSubscriptionAction / SaveSubscriptionAction.

No production code changes — test-only PR.

New test categories:

• Default action behavior — verifies omitting the action param defaults to "Create"
• Form field population — asserts all 6 fields (action, host, username, password, type, autoConnect) are correctly copied from Subscription → SubscriptionForm on Edit and Delete
• Delete with non-existing subscription — confirms redirect (previously only tested for Edit)
• Session attribute lifecycle — verifies SUBSCRIPTION_KEY is stored on edit and removed after save, cancel, and delete across all action types (5 tests)
• AutoConnect handling — explicit true, implicit default false, and update-on-edit paths
• Multiple simultaneous validation errors — all 4 required fields blank at once
• User model attribute on error paths — ensures user is in the model when returning the form with validation errors or duplicate-host errors (needed for template rendering)

Review & Testing Checklist for Human

• Verify session cleanup tests match intended controller behavior — do we actually want SUBSCRIPTION_KEY removed from the session on every save/cancel/delete path? Cross-reference with original SaveSubscriptionAction behavior.
• Confirm the autoConnect default-to-false test (saveSubscription_CreateAction_WithoutAutoConnect_ShouldDefaultToFalse) reflects correct Struts-era semantics — original used PropertyUtils.copyProperties which may have behaved differently for unchecked checkboxes.
• Spot-check that the new form-population tests (ShouldPopulateAllFormFields, ShouldPopulateFormFieldsFromSubscription) aren't redundant with the existing ShouldPopulateForm / ShouldShowForm tests — the new ones add field-level assertions the originals lacked.

Suggested test plan: cd apps/faces-example2 && mvn test — all 103 tests should pass.

Notes

• Builds on merged PR Phase 3: SubscriptionController + SubscriptionForm migration #21 which added @NotBlank password validation and baseline controller tests.
• Uses AssertJ (assertThat) for the new field-level and session assertions, consistent with Spring Boot test conventions. The existing tests used only MockMvc matchers.

Link to Devin session: https://jack-meigel.devinenterprise.com/sessions/8d11037d5a16405f975921abf07218fc
Requested by: @cogjack

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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