phsl / new-api

* fix(relay): treat gpt-5 and later generations alike for max_completion_tokens

IsOpenAIGPT5Model matched on the literal prefix "gpt-5", so gpt-6-astra
(and every generation after it) fell through the gpt-5 request rules:
max_tokens was forwarded as-is and the provider rejected it with
"Unsupported parameter: 'max_tokens' is not supported with this model.
Use 'max_completion_tokens' instead." The same gap left temperature,
top_p and logprobs untouched, each of which the provider also rejects,
and made the channel test button report a 400 for a healthy deployment.

Match on the major version instead (gpt-<n>... with n >= 5). Callers are
unchanged: ConvertOpenAIRequest, GetSystemRoleName, buildTestRequest and
the health check all go through this one helper. buildTestRequest now
sends max_completion_tokens for these models directly instead of relying
on the later conversion. gpt-4.1, gpt-4o, gpt-oss, gpt-image and
gpt-realtime names still do not match.

Verified against Azure OpenAI gpt-6-astra (2026-09-03): with the old
prefix max_tokens / temperature / top_p / logprobs each returned 400,
while gpt-5.6-luna with the same payload returned 200.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016bwz8o5UeoRtrtDusKaayp

* fix(relay): separate OpenAI chat model compatibility rules

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Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: CaIon <i@caion.me>