fix: update priority pricing for gpt 5.5

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修正gpt 5.5 priority定价。定价来源：OpenAI 官方 Priority 定价表

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[Copilot]

Pull request overview

This PR updates billing to reflect the official OpenAI GPT‑5.5 Priority pricing, addressing stale priority rates from the pricing mirror and ensuring correct costs are applied in both dynamic and fallback pricing paths.

Changes:

• Updated GPT‑5.5 Priority per-token prices in the mirrored pricing JSON (including the dated model variant).
• Added a billing-layer override to enforce official GPT‑5.5 Priority pricing before channel overrides are applied.
• Added a unit test to validate GPT‑5.5 Priority pricing behavior against stale mirror data.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
backend/resources/model-pricing/model_prices_and_context_window.json	Updates GPT‑5.5 Priority token pricing values to match official rates.
backend/internal/service/billing_service.go	Applies an explicit GPT‑5.5 Priority pricing override when mirrored data is stale.
backend/internal/service/billing_service_test.go	Adds coverage asserting GPT‑5.5 uses the official Priority prices even if mirror data is incorrect.

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recheck

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