fix: make deprecated Client initializer backward-compatible

[shuofengzhang]

What changed

• Fixed the deprecated Client initializer so it correctly forwards parameters to TavilyClient.
• Added backward-compatible handling for three call styles:
  • Client(api_key="...")
  • Client({"api_key": "..."})
  • Client("...")
• Added regression tests covering keyword-arg and legacy-dict initialization paths.

Why

• The deprecated Client class currently passes a single positional argument directly to TavilyClient, which can mis-handle initialization data (for example, passing a dict as the API key).
• This change preserves backwards compatibility while ensuring deprecated usage remains functional and predictable during migration to TavilyClient.

Testing

• ✅ source .venv/bin/activate && pytest -q

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Note

Low Risk
Low risk: changes are limited to a deprecated wrapper constructor and add tests; core request/response logic is untouched.

Overview
Restores backward-compatible initialization for the deprecated Client wrapper by accepting Client(api_key=...), Client({"api_key": ...}), and Client("...") and forwarding them as proper keyword args to TavilyClient.

Adds regression tests asserting the DeprecationWarning is emitted and the API key is set correctly for keyword-arg and legacy-dict construction.

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