Add raw chat source download endpoint

[proobker]

Fixes #160

Adds an authenticated raw ChatSource download endpoint with owner validation and plain text attachment responses.

[Tushar-sonawane06]

@avishek0769 I have reviewed this pr and it correct as per mentioned in the pr and issue, here is my review, you once take final look and merge it,

1.Complete feature implementation: authenticated endpoint with proper ownership validation

2.Robust content handling: supports both vectorless (document tree) and vector (Qdrant) sources

3.Excellent error handling and edge-case coverage

4.Strong test suite with 4 scenarios covering success and failure paths

5.Secure: ownrship validation enforced before any content returned

6.Clean, readable code with defensive programming

[avishek0769]

@proobker Yeah it looks good but please can you also integrate the api endpoint in the frontend too. Then I will label this as hard

[avishek0769]

@proobker Make the changes mentioned above and also resolve the merge conflicts