fix: enable dotenv override for environment variables

[Sumit189]

Problem

When a user has a stale or invalid API key exported in their shell profile
(e.g. export OPENAI_API_KEY=... in ~/.zshrc), the project's .env file
is silently ignored because load_dotenv() defaults to override=False.

This makes the error message misleading, it suggests adding the key to
.env, but that doesn't actually fix anything when a shell export exists:
Error: Authentication failed — your API key is missing or invalid. ... You can also add it to a .env file in the project root.

The user ends up stuck: the key works with curl, the .env looks correct,
but ml-intern keeps rejecting it with no indication that a shell variable
is shadowing the .env value.

Fix

Pass override=True to the project-root load_dotenv() call so the .env
file is the source of truth for API keys, matching the behavior the error
message already promises.

-    load_dotenv(_PROJECT_ROOT / ".env")
+    load_dotenv(_PROJECT_ROOT / ".env", override=True)
     load_dotenv(override=False)