fix(core/voice): honor OpenAI provider options over default clients

[green3sf]

Summary

This pull request fixes both OpenAIProvider and OpenAIVoiceModelProvider silently ignoring provider-specific client options when a global default OpenAI client is configured.

For OpenAIProvider, explicit api_key, base_url, websocket_base_url, organization, or project values now select a provider-specific client. For OpenAIVoiceModelProvider, explicit api_key, base_url, organization, or project values do the same.

Explicit empty values are preserved instead of falling through to global credentials or endpoint environment variables. Providers with all construction options omitted continue to reuse the exact global default client, and the existing explicit-client acceptance and conflict behavior remains unchanged.

Test plan

• Ran the focused provider, voice-provider, and configuration suites: 56 passed.
• Confirmed the 8 new voice regression cases fail against the unmodified merge-base source and pass with this patch.
• Ran .agents/skills/code-change-verification/scripts/run.sh; formatting, linting, type checking, and the full test suite all passed.

Issue number

None.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass
• If using Codex, I've run /review before submitting this PR