Add unit tests

[TisaneFruitRouge]

What

Adds a comprehensive unit test suite (267 tests across 9 files) covering enums, models, parsers, exporters, exceptions, transport, session, and fetcher layers. Also fixes a bug in _normalize_timeout
where httpx.Timeout(connect=a, read=b) raised a ValueError on httpx 0.28 — the call now passes a positional default: httpx.Timeout(b, connect=a, read=b).

AI Provenance

• Tool: Claude Code
• Model: Claude Opus 4.7
• Prompt/thread: The session began with a security audit of the repository before any code was written. Once the codebase was cleared as safe, dependencies were installed and the user requested thorough
  test coverage. The model first performed a full codebase exploration — reading every source file under src/trendflow/, mapping all classes, methods, return types, exception paths, and mock boundaries —
  before writing a single test. Tests were then written bottom-up (pure helpers first, then models, parsers, exporters, exceptions, transport, session, fetcher, and public API), with shared fixtures
  extracted into conftest.py. After the first full run, two categories of failures emerged: a missing RegionalInterestRow export from the package root (fixed in the test import), and a real source-level
  incompatibility in _normalize_timeout with httpx 0.28. The bug was diagnosed from the traceback, confirmed against the httpx API, and patched in the source. All 267 tests pass on Python 3.14.

Checklist

[X] Addresses exactly one issue or feature
[X] New or changed behavior has test coverage
[X] Diff contains only changes for this task

[TisaneFruitRouge]

Run the tests using uv run pytest

[dariomory]

@TisaneFruitRouge can you check the CI?

[TisaneFruitRouge]

@TisaneFruitRouge can you check the CI?

Sure