test(gateway): add tests for content-based session hash fallback
- 20 unit tests for deriveOpenAIContentSessionSeed covering:
- Empty/nil inputs, model-only, stable across turns
- Different model/system/first-user produce different seeds
- Tools, functions, developer role, structured content
- Responses API: input string, input array, instructions, input_text typed items
- JSON canonicalization (whitespace/key-order insensitive)
- Prefix presence, empty tools ignored, messages preferred over input
- 3 integration tests for GenerateSessionHash content fallback:
- Content fallback produces stable hash
- Explicit signals override content fallback
- Empty body still returns empty hash
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