Cherry-picked from PR #9359 by @luyao618.
- Accept camelCase aliases (apiKey, baseUrl, apiMode, keyEnv, defaultModel,
contextLength, rateLimitDelay) with auto-mapping to snake_case + warning
- Validate URL field values with urlparse (scheme + netloc check) — reject
non-URL strings like 'openai-reverse-proxy' that were silently accepted
- Warn on unknown keys in provider config entries
- Re-order URL field priority: base_url > url > api (was api > url > base_url)
- 12 new tests covering all scenarios
Closes#9332