feat: Immediately exit if configuration is missing or invalid

[rolodato]

This is the behaviour when running the Edge Proxy with no configuration:

% docker run --rm flagsmith/edge-proxy

2025-03-14T18:43:17.793837Z [info     ] Started server process [1]     [uvicorn.error]
2025-03-14T18:43:17.793915Z [info     ] Waiting for application startup. [uvicorn.error]
2025-03-14T18:43:17.795896Z [info     ] Application startup complete.  [uvicorn.error]
2025-03-14T18:43:17.796444Z [info     ] Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit) [uvicorn.error]
2025-03-14T18:43:18.033553Z [info     ] HTTP Request: GET https://edge.api.flagsmith.com/api/v1/environment-document/ "HTTP/1.1 401 Unauthorized" [httpx]
2025-03-14T18:43:18.034012Z [error    ] error_fetching_document        [edge_proxy.environments] client_side_key=environment_key
Traceback (most recent call last):
  File "/app/src/edge_proxy/environments.py", line 61, in refresh_environment_caches
    environment_document = await self._fetch_document(
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/src/edge_proxy/environments.py", line 150, in _fetch_document
    response.raise_for_status()
  File "/opt/venv/lib/python3.12/site-packages/httpx/_models.py", line 761, in raise_for_status
    raise HTTPStatusError(message, request=request, response=self)
httpx.HTTPStatusError: Client error '401 Unauthorized' for url 'https://edge.api.flagsmith.com/api/v1/environment-document/'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/401

On startup, we're always making an invalid request to fetch the environment document with ID environment_key. This is confusing and unnecessary.

Instead, we now validate environment key pairs using Pydantic so that any configuration errors immediately exit with a descriptive error. For example:

% rye run edge-proxy-serve                                                                            96.498s (feat/logging) 15:46
Traceback (most recent call last):
  File "/Users/rolodato/source/flagsmith/edge-proxy/.venv/bin/edge-proxy-serve", line 8, in <module>
    sys.exit(serve())
             ^^^^^^^
  File "/Users/rolodato/source/flagsmith/edge-proxy/src/edge_proxy/main.py", line 7, in serve
    settings = get_settings()
               ^^^^^^^^^^^^^^
  File "/Users/rolodato/source/flagsmith/edge-proxy/src/edge_proxy/settings.py", line 146, in get_settings
    return AppConfig()
           ^^^^^^^^^^^
  File "/Users/rolodato/source/flagsmith/edge-proxy/.venv/lib/python3.12/site-packages/pydantic_settings/main.py", line 84, in __init__
    super().__init__(
  File "/Users/rolodato/source/flagsmith/edge-proxy/.venv/lib/python3.12/site-packages/pydantic/main.py", line 176, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
pydantic_core._pydantic_core.ValidationError: 2 validation errors for AppConfig
environment_key_pairs.0.server_side_key
  String should match pattern 'ser\.*' [type=string_pattern_mismatch, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/string_pattern_mismatch
environment_key_pairs.0.client_side_key
  String should have at least 1 character [type=string_too_short, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/string_too_short