fix: descriptive error for unrecognized cell type attribute

[nkuprins]

Purpose of the pull request

Closed: #955

What's changed?

An .xlsx cell whose t attribute isn't a recognized type (s, str, inlineStr, e, b, n) made CellDataTypeEnum.buildFromCellType return null, which CellTagHandler passed straight into new ReadCellData<>(type), leading to IllegalArgumentException: Type can not be null, which names neither the cell nor the attribute.

CellTagHandler.startElement now detects the null and throws an ExcelAnalysisException that names the type and the cell:

Invalid cell data type: 'unknown' in cell B4

Following the discussion with @bengbengbalabalabeng on the issue, the check lives in the handler (rather than centralised in CellDataTypeEnum.buildFromCellType), so the message can point at the exact cell. And CellDataTypeEnum.buildFromCellType Javadoc now documents that it may return null for an unrecognised type.

Covered by CellTagHandlerTest.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[Copilot]

Pull request overview

This pull request improves diagnostics when reading .xlsx files by detecting unrecognized cell t attribute values during SAX parsing and throwing a descriptive ExcelAnalysisException that includes both the invalid type and the cell reference (addressing issue #955).

Changes:

• Add a null-check in CellTagHandler.startElement for CellDataTypeEnum.buildFromCellType(...) and throw an ExcelAnalysisException with cell/type context when the type is unrecognized.
• Document in CellDataTypeEnum.buildFromCellType Javadoc that the method may return null for unknown t values (and EMPTY for missing/empty t).
• Add a regression unit test ensuring the thrown error message includes both the unknown type and the cell reference.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
fesod-sheet/src/main/java/org/apache/fesod/sheet/analysis/v07/handlers/CellTagHandler.java	Throws a descriptive ExcelAnalysisException when an unknown cell t value would otherwise lead to a null type and a confusing downstream exception.
fesod-sheet/src/main/java/org/apache/fesod/sheet/enums/CellDataTypeEnum.java	Clarifies contract: may return null for unrecognized t values, EMPTY when t is empty/missing.
fesod-sheet/src/test/java/org/apache/fesod/sheet/analysis/v07/handlers/CellTagHandlerTest.java	Adds regression coverage verifying the new exception message includes both invalid type and cell reference.

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