Add tests/__init__.py to fix mypy duplicate-module error

[randileeharper]

Summary

Fixes #82.

Running mypy vesper tests failed immediately with:

tests/conftest.py: error: Source file found twice under different module names: "conftest" and "tests.conftest"

because tests/ had no __init__.py, so mypy resolved conftest under two module names. The test suite already imports from tests.conftest import ... (in test_historian.py and test_mcp.py), so treating tests as a package is the intended design — this makes it explicit, which is exactly the "simplest" option mypy itself recommends and the issue lists first.

This change was already in effect for runtime/pytest; it only corrects mypy's module resolution. After this change mypy no longer aborts with the duplicate-module error, and the production code (mypy vesper) remains fully clean (0 issues).

Follow-up

Fixing the duplicate-module error unmasked 51 pre-existing type errors in the test suite that mypy previously never reached (it aborted at the duplicate-module error before checking the test files). These are tracked separately in a follow-up issue and are out of scope here — this PR fixes the reported bug only.

Verification

Exact commands run (per AGENTS.md, using the project virtualenv):

.venv/bin/python -m mypy vesper tests      # duplicate-module error gone; vesper clean (51 pre-existing test errors remain, tracked separately)
.venv/bin/python -m mypy vesper           # Success: no issues found in 37 source files
.venv/bin/python -m pytest -q            # 251 passed
.venv/bin/python -m ruff check vesper tests   # All checks passed!
.venv/bin/python -m compileall -q vesper tests

Closes #82.