fix: raise LangfuseAuthCheckError instead of bare Exception in auth_check()

[mittalpk]

Fixes #906

What does this PR do?

Langfuse.auth_check() raised a bare Exception when no project was found for the provided credentials. A bare Exception is easy to swallow accidentally with a broad except Exception: at the call site (a common defensive-coding pattern in application code), which hides a genuine "your API keys don't correspond to a project" failure instead of letting the caller distinguish it from other errors — exactly the scenario described in #906.

• Added LangfuseAuthCheckError(Exception) in langfuse/_client/client.py, exported it publicly in langfuse/__init__.py (following the existing precedent of RegressionError from experiment.py).
• auth_check() now raises LangfuseAuthCheckError instead of a bare Exception in the "no project found" case; updated the docstring accordingly.
• This is purely additive/backwards-compatible: LangfuseAuthCheckError subclasses Exception directly, so any existing except Exception: call site still catches it.

Type of change

• Bug fix

Verification

uv run --frozen ruff check .
uv run --frozen ruff format --check .
uv run --frozen mypy langfuse --no-error-summary
uv run --frozen pytest -n auto --dist worksteal tests/unit

All clean; full unit suite passing with no regressions. Added tests/unit/test_auth_check.py covering both the new exception being raised when no projects are found, and the happy path being unaffected. Confirmed the new test fails against the pre-fix code (ImportError: cannot import name 'LangfuseAuthCheckError') before the fix is applied.

Checklist

• I self-reviewed the diff using code_review.md.
• I added or updated tests for behavior changes.
• I updated docs, examples, or .env.template if needed. (docstring updated; no example/.env.template changes needed)
• I did not hand-edit generated files; if generated files changed, I used the upstream regeneration path.
• I did not commit secrets or credentials.

Greptile Summary

This PR introduces a specific error for failed authentication checks. The main changes are:

• Adds and publicly exports LangfuseAuthCheckError.
• Raises the new error when credentials return no projects.
• Adds tests for empty and non-empty project responses.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

_{Reviews (1): Last reviewed commit: "fix: raise LangfuseAuthCheckError instea..." | Re-trigger Greptile}

Context used:

• Rule used - Move imports to the top of the module instead of p... (source)

Learned From
langfuse/langfuse-python#1387

[claude]

Claude Code Review

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