[Daksha1611]

What type of PR is this?

/kind bug

What this PR does / why we need it:

/v1/download_model and /v1/load_lora_adapter document config as a JSON object, but any non-empty object returns HTTP 500 because load_config passes it to json.loads.

This PR makes load_config accept a dict as well as a JSON string. The CLI continues to pass a string. The dict is copied so that the request body is not modified. Values that are not objects now raise ValueError with a clear message.

Which issue(s) this PR fixes:
Fixes #1976

Bug evidence (required for bug-related PRs):

Requests sent over HTTP to the runtime server started with python -m kthena.runtime.app, using a local pvc:// source.

config value	Before (3809f22)	After
{}	200	200
{"hf_token":"dummy"}	500, nothing downloaded	200, file downloaded
"{\"hf_token\":\"dummy\"}"	200	200
123	500, the JSON object must be str, bytes or bytearray, not int	500, Invalid configuration JSON format.

Error before the fix:

{"detail":"Download failed: the JSON object must be str, bytes or bytearray, not dict"}

This was reproduced locally and not on a cluster. /v1/load_lora_adapter was covered through the FastAPI test client rather than a running server.

Special notes for your reviewer:

A value that is not an object still returns HTTP 500 because the handlers map every exception to 500. Returning 400 would be a separate change, and I have kept it out of this PR.

Testing: pytest python/kthena/tests (125 passed, Python 3.12) and ruff check . --config python/kthena/pyproject.toml. Eight of the new tests fail without the fix.

Does this PR introduce a user-facing change?:

Fixed the runtime download endpoints returning HTTP 500 when `config` is a non-empty JSON object.

[volcano-sh-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[volcano-sh-bot]

Welcome @Daksha1611! It looks like this is your first PR to volcano-sh/kthena 🎉