fix: fall back to default format index on malformed style index (#355)

[Mikkey-f]

Fixes #355

What changes were proposed in this pull request?

CellTagHandler parses a cell's s attribute (style index) with a bare Integer.parseInt. A non-numeric value — as produced by some third-party tools — threw NumberFormatException and aborted the whole file read at the first corrupt cell (in the report: reading stopped at row 3218 of 3000+ rows).

The parse now falls back to the default format index (0) on NumberFormatException: a corrupt style index only loses that cell's style info, while the cell value and all subsequent rows are still read. This follows the reporter's suggestion and matches POI's lenient behavior (POI reads the same file completely, with the corrupt cell read as empty).

How was this patch tested?

• CellTagHandlerTest (2 new unit tests): non-numeric s falls back to dataFormatData(0) without throwing; numeric s keeps the exact lookup.
• CorruptStyleIndexReadTest (new end-to-end test): a minimal hand-built xlsx whose middle row carries s="abc" is read completely — all 3 rows returned.
• Red/green check: on the pre-fix code the e2e test fails with the exact exception from the report (NumberFormatException: For input string: "abc"); with the fix it passes.
• Full fesod-sheet suite: 915/915 green. spotless:check and javadoc pass.

Notes

• Zero impact on valid files: the numeric parse path is untouched.
• No API change, no new dependencies.