fix: resolve api_key from env var with runtime assertion

[kraenhansen]

Summary

• Prepares the wrapper client for SDK regeneration (🌿 Fern Regeneration -- April 7, 2026 #761) which makes api_key required in the generated base client
• Adds a _resolve_api_key() helper that resolves from the parameter or ELEVENLABS_API_KEY env var, raising a clear ValueError if neither is set
• Only touches src/elevenlabs/client.py (in .fernignore) and tests/test_client_init.py — no generated code modified
• Public API stays backwards-compatible: api_key remains Optional[str] on ElevenLabs and AsyncElevenLabs
• Also fixes a pre-existing bug: the old api_key=os.getenv("ELEVENLABS_API_KEY") default was evaluated once at import time, so env var changes after import were silently ignored. The env var is now read fresh on every client construction.

Mirrors the approach from elevenlabs-js#368.

Test plan

• ElevenLabs(api_key="key") — works as before
• ElevenLabs() with ELEVENLABS_API_KEY env var set — resolves from env
• ElevenLabs() without env var — raises clear ValueError
• Unit tests for _resolve_api_key, ElevenLabs, and AsyncElevenLabs (11 tests)
• Existing tests pass (webhooks, recursive models)
• Verify compatibility after merging 🌿 Fern Regeneration -- April 7, 2026 #761 (regeneration makes api_key: str required in base)

🤖 Generated with Claude Code

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Note

Medium Risk
Changes how ElevenLabs/AsyncElevenLabs obtain and validate API keys, which can break callers that relied on empty/missing keys or import-time env evaluation. Risk is limited to client construction/auth header setup and is covered by new unit tests.

Overview
API key handling is now resolved at runtime. ElevenLabs and AsyncElevenLabs no longer default api_key from os.getenv(...) at import time; instead they call a new internal _resolve_api_key() on construction to use the passed value or fall back to ELEVENLABS_API_KEY.

Missing/empty keys now fail fast. _resolve_api_key() raises a clear ValueError when neither an explicit key nor a non-empty env var is available (and treats "" as invalid), with new tests validating precedence and error cases; a convai import test was updated to stop passing an empty key.

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[kraenhansen]

Closing this as obsolete with https://github.com/elevenlabs/elevenlabs-dx/pull/2097 merged.