Refactor: consolidate duplicated _elapsed_ms and remove one-liner wrapper

[tommed]

Summary

• _elapsed_ms() was copy-pasted identically into all six validation check modules. Extracted to a new validation/_utils.py and replaced each local definition with an import.
• BacktestEngine._parse_zone() was a one-line method that only called the module-level _parse_zone() with self._products[0]. Inlined and removed.

Changes

src/nexa_backtest/validation/_utils.py (new file)
Shared home for _elapsed_ms(start: float) -> int — the single source of truth.

6 validation modules (ruff_check, mypy_check, interface_check, lookahead_check, feature_check, resource_check)
Removed local _elapsed_ms definition, added from nexa_backtest.validation._utils import _elapsed_ms.

src/nexa_backtest/engines/backtest.py
Removed BacktestEngine._parse_zone() wrapper method. Inlined _parse_zone(self._products[0]) at the one call site.

Test plan

• make ci passes — 727 tests, ruff check clean, ruff format clean

https://claude.ai/code/session_01XqYtc2t1oCV8eTmXN8aaiY

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR reduces duplication in the validation subsystem by extracting a shared _elapsed_ms() helper into a single module, and simplifies the backtest engine by removing an unnecessary one-line wrapper method.

Changes:

• Added validation/_utils.py as the shared home for _elapsed_ms(start: float) -> int.
• Updated all six validation check modules to import _elapsed_ms instead of defining it locally.
• Inlined BacktestEngine._parse_zone() usage at the only call site and removed the wrapper method.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/nexa_backtest/validation/_utils.py	Introduces a shared _elapsed_ms() helper used across validation checks.
src/nexa_backtest/validation/ruff_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/validation/mypy_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/validation/interface_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/validation/lookahead_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/validation/feature_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/validation/resource_check.py	Replaces local _elapsed_ms implementation with an import from _utils.
src/nexa_backtest/engines/backtest.py	Inlines _parse_zone(self._products[0]) and removes the redundant wrapper method.

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