test: expand test coverage with edge cases, error paths, and shared conftest

[dgenio]

What changed

Closes #6 — expand test coverage for edge cases, error paths, and context accumulation.

New test cases in tests/test_flow_execution.py

• TestSingleStepFlow — one-step flow with input_mapping (simplest chaining case)
• TestContextAccumulation — verifies that outputs from all steps (not just the last) are merged into final_output, including the original initial_input keys
• TestToolZeroDivisionError — ZeroDivisionError inside a tool fn is wrapped as FlowExecutionError
• TestBoundaryValues — negative numbers, zero, and large negatives through the double→add→format chain

New test cases in tests/test_registry.py

• test_empty_registry_returns_none — match_flow_by_intent on an empty registry returns None
• TestOverwritePreservesCount — len() stays the same after register_flow(..., overwrite=True)

Structural improvements

• tests/conftest.py (new) — shared Pydantic schemas (NumberInput, ValueOutput, ValueInput, FormattedOutput), helper functions (_double_fn, _add_ten_fn, _format_fn), and pytest fixtures (double_tool, add_ten_tool, format_tool, linear_flow, executor) extracted from test_flow_execution.py
• tests/__init__.py removed — allows pytest to auto-add the test directory to sys.path so test modules can import from conftest.py
• pyproject.toml — added pytest-cov>=4.0 to [project.optional-dependencies] dev

Note on issue scope

The issue listed test_empty_flow_succeeds — this test already existed in TestEmptyFlow so no duplicate was added.

Why

Missing coverage for edge cases (single-step, boundary values, context accumulation) and specific exception types (ZeroDivisionError) reduced confidence for the v0.1.0 release. Shared fixtures reduce duplication across test files.

How verified

pytest tests/ -v --cov=chainweaver --cov-report=term-missing  → 46 passed, 0 failed
ruff check chainweaver/ tests/ examples/                       → All checks passed
ruff format --check chainweaver/ tests/ examples/              → 12 files already formatted

Coverage: 99% (213 statements, 3 missed)

Module	Cover	Missing
__init__.py	100%	—
exceptions.py	100%	—
executor.py	99%	L204
flow.py	100%	—
log_utils.py	100%	—
registry.py	96%	L100
tools.py	94%	L88

Tradeoffs / risks

• Removing tests/__init__.py changes how pytest discovers tests (rootdir-based instead of package-based). This is the standard pytest recommendation and has no practical impact for this project.
• No existing tests were modified beyond import deduplication (schemas/fixtures moved to conftest).

[Copilot]

Pull request overview

This PR expands test coverage for the chainweaver package by adding new test cases for edge cases and error paths, and refactors shared test infrastructure into a central conftest.py.

Changes:

• New test cases in test_flow_execution.py for single-step flows, context accumulation, ZeroDivisionError wrapping, and boundary values (negative numbers, zero, large negatives)
• New test cases in test_registry.py for empty-registry intent matching and overwrite-preserves-count behavior
• Shared Pydantic schemas, helper functions, and pytest fixtures extracted from test_flow_execution.py into a new tests/conftest.py; tests/__init__.py removed; pytest-cov>=4.0 added to dev dependencies

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_flow_execution.py	Removes in-file fixtures/schemas (moved to conftest); adds TestSingleStepFlow, TestContextAccumulation, TestToolZeroDivisionError, and TestBoundaryValues
tests/test_registry.py	Adds test_empty_registry_returns_none to TestMatchFlowByIntent and a new TestOverwritePreservesCount class
tests/conftest.py	New file with shared Pydantic schemas, tool functions, and pytest fixtures for all test files
tests/__init__.py	Removed to enable rootdir-based pytest discovery (allows from conftest import in test files)
pyproject.toml	Adds pytest-cov>=4.0 to dev optional dependencies

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