Preserve carriage returns in XLSX string values

[levvs-one]

Summary

The XLSX string encoder does not currently escape CR (U+000D). The XML writer's newline handling then changes CR and CRLF to LF, so writing and reading a string such as "a\rb" does not preserve the original value.

This change adds CR to the existing OpenXmlCodec escape regex, producing _x000d_ before XML serialization. It keeps the current codec, literal-escape protection and public APIs unchanged.

Tests

• Added encode/decode and real XLSX writer/reader regressions for CR, CRLF, LF, a leading CR, and literal/nested escape sequences.
• With the new tests but before the fix: 22 passed, 6 failed.
• With the fix: the same filter passes all 28 tests.
• Full test project on Windows x64, net8.0 using SDK 10.0.400: 363 passed, 0 failed, 3 existing skipped tests (366 total). SylvanExcelTestData was unset, so the optional external corpus was not exercised.
• A separate .NET 10 consumer verified exact string roundtrips and inspected the generated XLSX with Open XML SDK and BCL ZIP/XML APIs. Desktop Excel and Linux/AOT were not tested.

The patch only changes CR escaping in the existing XLSX path; it does not change reader whitespace policy or add a different string codec.