Threads endpoints: credentials fetching refactoring

[nishika26]

Summary

Target issue is #210
Explain the motivation for making this change. What existing problem does the pull request solve?
for #210 -
-Langfuse credentials are now optional, allowing the endpoint to function even if Langfuse keys are not configured.

-This change provides more flexibility in the API by making OpenAI credentials the only required ones, while still supporting Langfuse if it’s available.

###NOTES

• Added explicit tracing enabling and disabling of Langfuse configuration during setup. This ensures that Langfuse is correctly configured only for users with valid credentials, preventing it from remaining active and showing traces when switched to users without Langfuse credentials - referred this from here

[AkhileshNegi]

@nishika26 did you test it? It should work like this:

1. With both Langfuse and OpenAI creds: it runs and you see traces in Langfuse.
2. With only OpenAI creds: it runs, but no traces in langfuse
3. With neither set: it fails and throws error missing OpenAI credentials.

[AkhileshNegi]

Also fetching credentials from database, is this implemented across modules? Like even with collections endpoint and file upload?

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/app/api/routes/threads.py	25.00%	6 Missing ⚠️
backend/app/core/util.py	14.28%	6 Missing ⚠️

📢 Thoughts on this report? Let us know!

[avirajsingh7]

@nishika26 @AkhileshNegi will be closing this PR due to #249,
As I am working on langfuse for thread/response and changing the way it is implemented will take of this also when raising a MR.