Harden user-state startup-mode composition (backend-3.5.5)

[leynos]

Summary

• Implements the approved execplan for backend roadmap item 3.5.5: docs/execplans/backend-3-5-5-harden-startup-mode-composition.md.
• Hardens backend/src/server/state_builders.rs by routing user-state startup composition through an explicit private helper seam and port bundle, keeping DB-present versus fixture-fallback adapter selection deterministic as wiring evolves.
• Strengthens startup-mode behavioural coverage with shared rstest-bdd assertions for DB-backed and fixture-backed profile, users-list, and preferences responses.
• Records the design decision and testing convention in the backend architecture and developer guides, and marks roadmap item 3.5.5 done.

Validation

• cargo test -p backend --test startup_mode_composition_bdd -- --nocapture
• make check-fmt
• make lint
• make test
• coderabbit review --agent after the helper-seam milestone: 0 findings.
• coderabbit review --agent after the regression-assertion milestone: 0 findings.
• coderabbit review --agent after the documentation milestone: 0 findings.

Notes

• docs/users-guide.md was not updated because this change does not alter operator-visible server behaviour or user interface.
• Property, Kani, and Verus coverage were not added because the change preserves existing startup adapter selection behaviour rather than introducing a new ranged invariant, state machine, or proof-bearing business rule.

References

• Execplan: docs/execplans/backend-3-5-5-harden-startup-mode-composition.md
• Lody session: https://lody.ai/leynos/sessions/f407c9df-ba22-40a7-842e-5e0eb11778b9

[sourcery-ai]

Reviewer's Guide

Pre-implementation ExecPlan for roadmap item 3.5.5 has been substantially rewritten and tightened: it now formalises an approval gate, narrows scope to hardening user-state startup-mode composition, updates constraints/tolerances/risks, describes current code/test context more concretely, and lays out a staged implementation and validation plan without changing any production code.

File-Level Changes

Change	Details	Files
ExecPlan purpose, status, and observable-success criteria refocused on user-state startup-mode composition with an explicit approval gate before implementation.	Clarified that this PR is a pre-implementation planning change and must be approved before any code work begins. Reframed the purpose around keeping DB-present vs fixture-fallback user-state adapter selection deterministic as wiring evolves. Replaced earlier success criteria with ones that emphasise explicit user-state helper seams, deterministic adapter selection, and rstest/rstest-bdd coverage rather than specific prior stage labels.	docs/execplans/backend-3-5-5-harden-startup-mode-composition.md
Constraints, tolerances, risks, and ownership sections rewritten to align with current architecture guidelines, testing stack, and agent usage model.	Constraints now emphasise preserving hexagonal architecture invariants, keeping persistence in outbound adapters, avoiding new dependencies, and using rstest/rstest-bdd according to existing guides. Tolerances now bound scope and churn (files/LOC), restrict interface and dependency changes, and add behaviour/validation/architecture tolerances for startup-mode behaviour. Risks have been updated to focus on HttpStateExtraPorts defaulting, repeated db_pool branching, route-submission composition location, test flakiness with embedded Postgres, and avoiding overlap with roadmap 3.5.6. Replaced the earlier multi-agent implementation roles with a narrower agent-team description that keeps this PR strictly planning-only.	docs/execplans/backend-3-5-5-harden-startup-mode-composition.md
Context, plan-of-work, validation, and artefact sections restructured into a staged, command-level implementation roadmap without introducing any code changes.	Expanded context to enumerate the current composition root, key helper functions, HTTP state types, existing startup-mode tests, and supporting docs that will be touched later. Replaced earlier stage descriptions with new stages A–D that cover approval, helper seam introduction, behavioural regression strengthening, documentation/roadmap closure, and detailed concrete steps and commands. Updated validation and acceptance to describe the required post-implementation properties (deterministic adapter selection, error/validation envelope stability, test coverage, gates, and CodeRabbit review). Reworked artefact/notes, interfaces/dependencies, and idempotence/recovery sections to clarify required logs, helper-shape expectations, and how to safely rerun or correct the work once implementation starts.	docs/execplans/backend-3-5-5-harden-startup-mode-composition.md

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