fix(async): preserve typed errors in research stream mode

[shuofengzhang]

What changed

• Fixed AsyncTavilyClient research streaming error handling so Tavily-specific exceptions are no longer wrapped into a generic Exception.
• Added explicit pass-through for:
  • UsageLimitExceededError
  • ForbiddenError
  • InvalidAPIKeyError
  • BadRequestError
  • TimeoutError
• Added a regression test that verifies async streaming research preserves UsageLimitExceededError on HTTP 429.

Insight / Why this matters

In async stream mode, _research(..., stream=True) raised domain-specific exceptions for non-200 responses, but then immediately caught them with a broad except Exception and re-raised a generic error.

That made stream-mode error handling inconsistent with non-stream mode and with the sync client path. It is easy to miss because happy-path stream tests pass, while typed error handling only appears in failure scenarios.

This change preserves typed exceptions, so callers can implement correct retry/backoff and user messaging logic based on error class instead of brittle string parsing.

Practical gain / Why this matters

• Improves production reliability for streaming consumers.
• Enables precise client behavior (e.g., handle quota limits differently from auth failures).
• Reduces integration bugs caused by generic exception wrapping.

Why

• Align async streaming behavior with the rest of the SDK’s error model.
• Keep failures actionable for SDK users in real integrations.

Testing

• ✅ pytest -q tests/test_research.py
• ✅ pytest -q

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Note

Medium Risk
Changes exception propagation in async streaming mode, which can affect downstream error handling, but is limited in scope and covered by a targeted regression test.

Overview
Async research(..., stream=True) now preserves Tavily-specific error classes (UsageLimitExceededError, ForbiddenError, InvalidAPIKeyError, BadRequestError, TimeoutError) instead of catching them and rethrowing a generic Exception; unexpected errors are still wrapped but now use exception chaining (raise ... from e).

Adds a regression test ensuring a 429 response in streaming mode raises UsageLimitExceededError with the server-provided message.

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