fix: check all errors in Phase C store tests

[Copilot]

Review feedback identified several places in the Phase C test suite where errors were silently discarded, masking root-cause failures and allowing tests to pass incorrectly.

Changes

• Setup calls — CreateCompany, CreateOrganization, CreateProject, CreateWorkflow in all Phase C tests now t.Fatalf on error instead of using _
• InsertExecution — errors now asserted in TestV1Store_ExecutionStepLifecycle and TestV1Store_ExecutionFailure
• Row iteration — rows.Scan() error checked inside loop; rows.Err() checked after loop in TestV1Store_ExecutionStepLifecycle
• CountExecutionsByWorkflow — error now asserted before inspecting the returned map in TestV1Store_ExecutionFailure

Before:

company, _ := store.CreateCompany("Co", "", "u1")
_ = store.InsertExecution(execID, wf.ID, "manual", "running", "u1", now)
counts, _ := store.CountExecutionsByWorkflow(wf.ID)

After:

company, err := store.CreateCompany("Co", "", "u1")
if err != nil {
    t.Fatalf("CreateCompany: %v", err)
}
if err = store.InsertExecution(execID, wf.ID, "manual", "running", "u1", now); err != nil {
    t.Fatalf("InsertExecution: %v", err)
}
counts, err := store.CountExecutionsByWorkflow(wf.ID)
if err != nil {
    t.Fatalf("CountExecutionsByWorkflow: %v", err)
}

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