fix: env overrides when config.json is missing and add regression test

[tbeaudouin05]

• Problem: When config.json is absent (common in Fly deployments using secrets/env only), LoadConfig returned defaults without applying env overrides, so the app still used the default model (glm-4.7) and failed due to missing credentials.
• Fix: Always run env.Parse(cfg) even when config.json is missing, so secrets/env values take effect in env-only deployments.
• Test: Added TestLoadConfig_EnvOverridesWithoutFile to verify env vars (e.g., Fly secrets for model/provider) are honored without a config file.

[mrbeandev]

Good catch on the LoadConfig logic! It seems like we're both looking at the parsing order of environment variables. I've actually just updated PR #239 which handles this by unsetting conflicting env vars at the entrypoint level (ensuring the JSON remains the source of truth) while also adding full Coolify/Docker support. It might be worth checking how we can unify these so they don't fight each other!

[Leeaandrob]

@Zepan Fixes env overrides when config.json is missing entirely, plus regression test. Companion to #270.

Recommendation: Merge. +19 lines with a test. Fixes the bootstrapping experience for new users who haven't created a config file yet.

[nikolasdehor]

Clean, minimal fix for an important edge case. In file-less deployments (Fly.io, Docker with env-only config), LoadConfig was returning DefaultConfig() without calling env.Parse(cfg), silently ignoring all environment variable overrides.

The regression test is well-written — uses t.Setenv for proper cleanup and filepath.Join(t.TempDir(), "missing.json") to guarantee a non-existent path. LGTM.