fix: add a special error type for configuration errors

[cpacker]

Cleans up some common error triggers on the server side:

• When the server is configured with a model but the API key doesn't exist, the client gets a confusing retry error instead of an API key error
• ADE constantly hits /agents routes for .../context, .../archival, but gets a 500 instead of the correct 404

---

Before:

Server-side:

INFO:     ::1:56137 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 200 OK
...
Exception: Retries exhausted and no valid response received.
None
Retries exhausted and no valid response received.
...
  File "/Users/loaner/miniconda3/envs/letta/lib/python3.12/site-packages/letta/agent.py", line 641, in _get_ai_reply
    raise Exception("Retries exhausted and no valid response received.")
Exception: Retries exhausted and no valid response received.

Client-side:

Traceback (most recent call last):
  File "/Users/loaner/miniconda3/envs/letta/lib/python3.12/site-packages/letta/client/client.py", line 975, in send_message
    raise ValueError(f"Failed to send message: {response.text}")
ValueError: Failed to send message: {"detail":"Retries exhausted and no valid response received."}

After:

Server-side:

INFO:     Finished server process [65818]
letta-py3.10(base) loaner@MacBook-Pro-8 MemGPT-fresh % letta server

[[ Letta server // v0.6.1 ]]
▶ View using ADE at: https://app.letta.com/development-servers/local/dashboard
▶ Server running at: http://localhost:8283

INFO:     Started server process [65971]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://localhost:8283 (Press CTRL+C to quit)
INFO:     ::1:59716 - "GET /v1/jobs/active HTTP/1.1" 200 OK
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/archival HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/jobs/active HTTP/1.1" 200 OK
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/jobs/active HTTP/1.1" 200 OK
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/archival HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/archival HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/jobs/active HTTP/1.1" 200 OK
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/archival HTTP/1.1" 404 Not Found
INFO:     ::1:59716 - "GET /v1/agents/agent-bbd31eda-8b95-46d3-957e-312ef4fe2911/context HTTP/1.1" 404 Not Found
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.