Fix ENTSO-e date format for startDateTimeUpdate parameter

[Copilot]

The startDateTimeUpdate date formatting used toISOString().replace(":00.000Z", ":00Z") which only strips seconds when they're exactly zero. Dates with non-zero seconds (e.g., 2024-01-15T10:30:45.123Z) would be sent to ENTSO-e with seconds/milliseconds intact, causing API issues.

Changes

• Extracted date formatting logic into a reusable helper module (src/helpers/date.ts)
• Updated both startDateTimeUpdate and startDateTime formatting to use the new helper
• Added comprehensive tests for the date formatting functionality (tests/date.test.ts)
• Ensures all dates are formatted as YYYY-MM-DDTHH:mmZ regardless of input seconds value

Before/After

// Before (broken for non-zero seconds)
new Date("2024-01-15T10:30:45.123Z").toISOString().replace(":00.000Z", ":00Z")
// → "2024-01-15T10:30:45.123Z" ❌

// After (always correct)
toEntsoeFormat(new Date("2024-01-15T10:30:45.123Z"))
// → "2024-01-15T10:30Z" ✓

Testing

• Added 7 tests covering:
  • Date objects with zero and non-zero seconds
  • ISO 8601 strings in correct format
  • Parseable date strings
  • Invalid strings and Date objects
  • Parameter name in error messages

Original prompt

Clean up the last commit, ensuring correct date format for entsoe requests

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[Hexagon]

@copilot, add tests for this date handling features if it doesnt already exist

[Hexagon]

@copilot make sure there are teests for this, also review if opther related tests are missing. Also review ci test failures.

[Copilot]

@copilot, add tests for this date handling features if it doesnt already exist

Added comprehensive tests in tests/date.test.ts covering:

• Date objects with zero and non-zero seconds
• ISO 8601 strings in correct format
• Parseable date strings
• Invalid inputs and error messages

Also refactored the date formatting into a reusable helper at src/helpers/date.ts (561dc53).

[Copilot]

@copilot make sure there are teests for this, also review if opther related tests are missing. Also review ci test failures.

Tests for date handling are present in tests/date.test.ts with 7 tests covering:

• Date objects formatting (with zero and non-zero seconds)
• ISO 8601 strings in correct format
• Parseable date strings conversion
• Invalid input handling
• Error message parameter names

All tests pass. The CI failure is unrelated to my changes - it's a "Bad credentials" error in the lcov-reporter-action coverage reporting step (561dc53).