feat: add set_langfuse_context helpers for trace metadata

[chakshu-dhannawat]

Adds convenience helpers to set/reset Langfuse trace-level context without manually touching the internal context variable.

New exports in haystack_integrations.tracing.langfuse:

• set_langfuse_context(trace_id, user_id, session_id, tags, version)
• reset_langfuse_context(token)
• LangfuseContextToken

These wrap the existing tracing_context_var plumbing that LangfuseTracer already reads when creating a root trace. They make it easier to attach user_id, session_id, tags, version, and trace_id to the next pipeline run, as discussed in #1279 and #1264.

Closes #1279
Relates to #1264

How did you test it?

• Added unit tests covering full/partial updates, token-based reset, and clearing.
• Ran hatch run test:unit and hatch run test:types for the langfuse integration.

Checklist

• I have read the contributors guidelines and the code of conduct
• I added unit tests and updated the docstrings

[github-actions]

Hi @chakshu-dhannawat, thanks for your interest in contributing to Haystack! 🙏

⚠️ You currently have 3 open pull requests in this repository (#3846, #3525 and this one). Our review capacity is limited, so please hold off opening more PRs until we've had a chance to review your first 2 open PRs. This helps us give each contribution the attention it deserves. Thank you!

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[github-actions]

Heads-up for maintainers

This PR is from a fork and touches integrations whose integration tests require API keys.
Those tests are skipped in CI because fork PRs don't have access to repo secrets for security reasons.

Affected integrations:

• langfuse

Please run the integration tests locally (hatch run test:integration inside each folder) before approving.

[github-actions]

Coverage report (langfuse)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/langfuse/src/haystack_integrations/tracing/langfuse
tracer.py
Project Total

This report was generated by python-coverage-comment-action

[chakshu-dhannawat]

Hi @julian-risch, gentle ping for a review when you have a moment. Thanks.