test(coverage): raise decision_api.lua and saas_client.lua coverage above 80%

[levleontiev]

Context

After PR #28 (issue #25), the following modules remain below 80% non-generated coverage:

File	Coverage
decision_api.lua	~58.87%
saas_client.lua	~78.88%

Goal

Add targeted unit tests to raise both files meaningfully above 80%.

decision_api.lua gaps (58.87%)

Key uncovered paths:

• build_request_context body parsing branches (JSON body, body file fallback)
• _resolve_ip_from_header multi-value/invalid header branches
• access phase: shadow-mode path, cost_based rule path
• _apply_llm_budget_rule branches
• body_filter_response non-streaming reconciliation path
• _get_header_hint_tokens edge cases

saas_client.lua gaps (~78.88%)

Key uncovered paths:

• queue_event when transport is unavailable
• Retry/backoff logic branches
• flush with empty queue
• HTTP error response handling

Acceptance criteria

• decision_api.lua coverage > 75% (from 58.87%)
• saas_client.lua coverage > 85% (from 78.88%)
• All existing + new tests pass (busted spec/unit/)
• lua-quality CI green