fix(diagnostics): use internal_api_base instead of hardcoded port 8000

[cyq1017]

Summary

The /api/test-research endpoint in routes/diagnostics_routes.py hardcoded the port 8000 (e.g. http://127.0.0.1:8000/v1/chat/completions). If the Odysseus server runs on a custom port or behind a reverse proxy, this endpoint fails.

This PR replaces the hardcoded endpoint with the dynamic internal_api_base() resolution helper. It also updates the regression tests in tests/test_internal_api_base.py to ensure that no hardcoded :7000 or :8000 are introduced in the future.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #3826

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app and verified the change works end-to-end.

How to Test

1. Run the Odysseus server on a custom port (e.g., 7000) using APP_PORT=7000 uvicorn app:app --port 7000.
2. Access the diagnostics test-research endpoint, and confirm it connects to the chat completions API successfully without timing out or resolving to port 8000.
3. Run the regression unit test:
   python3 -m pytest tests/test_internal_api_base.py

Visual / UI changes — REQUIRED if you touched anything that renders

N/A (Backend routes change only, no UI/visual changes)

[pewdiepie-archdaemon]

GitHub reports this PR is currently conflicting with dev, so I can’t review or approve it cleanly yet. Please rebase/update against current dev and rerun the focused diagnostics check once it is mergeable again.