[Enhancement] Improve test coverage and cleanup for EscapeHexCellWriteHandler

[nkuprins]

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• I searched in the issues and found nothing similar.

Motivation

While working on #991, I noticed that EscapeHexCellWriteHandler has no dedicated tests, and the pattern-scanning code has some edges worth tidying.

Solution

I will do it in 2 PR.

1. Testing

• Cover the escaping logic itself - valid patterns get rewritten, invalid/edge-case ones don't,
  and non-applicable inputs (wrong cell data type, null values) are safely ignored.
• Cover it end-to-end - write a file through Fesod with the handler registered and confirm the
  escaped value survives being read back.

2. Refactoring/cleanup

Hold off reviewing this section until 1. is merged - the tests need to pin current behaviour before
anything underneath them moves.

• Reduce duplication in the pattern-scanning code - the match-detection logic is currently
  written out twice across two methods.
• Improve readability of the low-level string offsets (magic numbers, member ordering), so the
  logic is easier to verify by inspection.

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[delei]

The unit test class might have been omitted when renaming the package. You can refer to the PR #470 and issue #364.

[nkuprins]

The unit test class might have been omitted when renaming the package. You can refer to the PR #470 and issue #364.

Confirmed! The accidental deletion happened in #770. Then, I guess I will restore it and see if something can be improved there.