OpenRouter fix

In a recent release we introduced the ability to change the User-Agent string, as well as setting a default User-Agent for Fantasy.

Well... looks like OpenRouter has specific behavior for the OpenAI Go SDK that otherwise break, so need to keep the original SDK User-Agent for OpenRouter specifically so it works correctly.

We also changed the default User-Agent a bit to better follow the convention:

• Previous: Charm Fantasy/v1.2.3
• Now: Charm-Fantasy/v1.2.3 (https://charm.land/fantasy)

Changelog

Fixed

• 96f0454: fix(openai): don't mutate shared languageModelOptions (@meowgorithm)
• 522e026: fix(openai): propagate noDefaultUserAgent to responsesLanguageModel (@meowgorithm)
• e8560e6: fix(openai): use noDefaultUserAgent in all responses API calls (@meowgorithm)
• 2e29858: fix(openrouter): avoid brittle User-Agent assertion (@meowgorithm)
• 7dc3888: fix(openrouter): don't set User-Agent (@meowgorithm)
• a3b8a69: fix(openrouter): preserve anthropic reasoning signature in streaming (@andreynering)
• 152acfe: fix: guard against nil EncryptedContent dereference in reasoning details (@andreynering)
• 50a1116: fix: improve default user-agent string (@andreynering)

Docs

• 182ba5f: docs(openai): correct grammar in comment (@meowgorithm)

Other stuff

• fa02069: v0.12.2 (@andreynering)

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