feat(providers): add Opper as an OpenAI-compatible gateway

[Felixkw12]

Opper is an EU hosted gateway. One key reaches models from OpenAI, Anthropic, Google, Mistral, DeepSeek, Moonshot and open weight vendors at https://api.opper.ai/v3/compat.

Mirrors the Requesty provider added in #159: a ProviderSpec on the generic openai_compat gateway path, the matching ProviderConfig block, and tests pinning the wiring.

Two provider specific decisions, both covered by tests. Prompt caching is on, because Opper serves Anthropic models and prices cached input separately, and bare pool names start with claude so the existing trigger applies. strip_model_prefix stays off, unlike Forge: Opper ids are bare pool names, but a provider/model id such as azure/gpt-5.5 deliberately pins one route, so stripping the prefix would silently change routing.

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Changes Made

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Checklist

• Changes tested locally
• Code reviewed
• Documentation updated (if necessary)
• Unit tests added (if applicable)

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Disclosure: I work at Opper. Opper is an OpenAI-compatible gateway hosted in the EU, and a good part of what it adds over a generic endpoint is European native routes, so requests can be pinned to EU-hosted regions. Happy to adjust anything here to match project conventions, or to close it if a named provider is not wanted.

[Zongwei9888]

Merged into main as ab088e6. Thank you @Felixkw12 — and thank you for the upfront disclosure, which made this easy to review. The template follows the Requesty shape from #159, and both provider-specific choices are well argued: prompt caching on because Opper prices cached Anthropic input separately, and provider/model prefixes kept because they pin a route. Your tests pass locally along with the full suite.

One small addition on our side: opper is now listed in the endpoint-class table in tests/test_provider_egress.py, so the egress policy's classification of the template is pinned like the other gateways.