Update REACH Download to use Date-Based API

[Alrobbertz]

• Switched the REACH import to the absolute, midnight-aligned UTC window API (download_UDL_reach_window), replacing the seconds-based REACH_DELAY_SECONDS/REACH_WINDOW_SECONDS with day-based REACH_WINDOW_END_DAYS_AGO and REACH_WINDOW_DAYS (defaults preserve the current 54h–30h-ago window) so output files reliably cover a full midnight-to-midnight day.
• Updated README.rst to document the new env vars and midnight-snapping behavior, and fixed the bucket vars (REACH_DESTINATION_BUCKET → _DEV/_PROD).
• Updated test_import_UDL_REACH_to_s3 to mock download_UDL_reach_window and assert midnight-aligned edges, a one-day-before-midnight end, and a one-day window span.
• Reworked buildspec.yml: removed the lint/test pre_build phase and parameterized the build to derive ECR account/repo and tag/version dynamically (unrelated to the REACH change).

closes #26

[codecov]

Codecov Report

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Additional details and impacted files

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[Copilot]

Pull request overview

Updates the REACH ingestion flow to use a date-based, UTC-midnight-aligned download window API, replacing the prior seconds-based delay/window configuration to ensure each run covers an exact midnight-to-midnight day.

Changes:

• Switch REACH download callsite to download_UDL_reach_window(start_time, end_time) and compute midnight-snapped UTC day windows from new env vars.
• Update unit test to mock download_UDL_reach_window and assert midnight-aligned window edges and one-day span.
• Rework buildspec.yml to parameterize ECR/account/tagging logic and remove the prior lint/test pre-build steps.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
README.rst	Documents the new day-based REACH window env vars and midnight-snapping behavior.
lambda_function/src/executor/executor.py	Computes UTC-midnight window boundaries and calls the new REACH window download API.
lambda_function/tests/test_executor.py	Updates REACH import test to validate midnight-aligned window semantics with the new API.
buildspec.yml	Refactors CodeBuild image build/push logic to use dynamic account/repo/version values.

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