fix(openai): merge custom-provider system prompts

[pandego]

Summary

• merge consecutive system and user messages for custom openai_chatcompletions providers
• align custom OpenAI-compatible providers with the existing DMR prompt-normalization behavior
• add regression coverage so custom providers collapse multiple system prompts while the native OpenAI path stays unchanged

Testing

• docker run --rm -v "$PWD":/src -w /src golang:1.26 sh -lc '/usr/local/go/bin/gofmt -w pkg/model/provider/openai/client.go pkg/model/provider/openai/client_test.go && /usr/local/go/bin/go test ./pkg/model/provider/openai ./pkg/model/provider/oaistream -run "TestConvertMessages_MergesConsecutiveSystemMessagesForCustomProviders|TestConvertMessages_PreservesConsecutiveSystemMessagesForOpenAIProvider|TestConvertMessages|TestMergeConsecutiveMessages" -count=1'

Closes #2327

[aheritier]

LGTM. Clean, minimal fix that reuses existing infrastructure (isCustomProvider, MergeConsecutiveMessages). Both paths are well-tested and CI passes.

One nit: the em dash → hyphen change on line 51 of the test file is unrelated noise — consider keeping commits atomic in the future.