fix: preserve explicit API key semantics - #778

Summary

• resolve DEEPGRAM_API_KEY at construction only when the api_key argument is omitted
• preserve the existing behavior where explicit api_key=None rejects authentication instead of silently using an ambient process credential
• isolate the import-time environment regression in a subprocess that starts without DEEPGRAM_API_KEY
• add sync and async coverage for explicit-None behavior

Follow-up to #767 and #734.

Why

#767 correctly fixed delayed environment loading, but its kwargs.get("api_key") is None check treated an omitted argument and explicit None as equivalent. In tests, multi-tenant services, or applications intentionally suppressing ambient authentication, explicit None must not activate a process-wide credential.

Verification

• full Docker-backed suite: 1020 passed, 1 skipped
• mypy src: clean across 901 source files
• Ruff: clean for both changed files
• seven-scenario sync/async credential probe: all passed
• regression tests proven non-vacuous against both the pre-fix: resolve DEEPGRAM_API_KEY when the client is constructed (closes #734) #767 baseline and the original fix: resolve DEEPGRAM_API_KEY when the client is constructed (closes #734) #767 implementation
• git diff --check: clean