Add per-sheet filtering to ExcelToPdfConverter (SheetIndices / SheetNames)

[Copilot]

ExcelToPdfConverter had no way to select a subset of sheets — it always converted every sheet. This adds two mutually-exclusive filter options to ConversionOptions.

API changes

ExcelToPdfConverter.ConversionOptions — two new optional properties:

Property	Type	Behaviour
SheetIndices	IReadOnlyList<int>?	Include only the specified zero-based sheet indices
SheetNames	IReadOnlyList<string>?	Include only sheets matching by name (case-insensitive)

SheetIndices takes precedence when both are set. null / empty → all sheets (existing default behaviour unchanged).

// By index
var options = new ExcelToPdfConverter.ConversionOptions
{
    SheetIndices = new[] { 0, 2 },   // first and third sheet only
};

// By name (case-insensitive)
var options2 = new ExcelToPdfConverter.ConversionOptions
{
    SheetNames = new[] { "Sales", "Summary" },
};

var doc = ExcelToPdfConverter.Convert("data.xlsx", options);

Changes

• ExcelToPdfConverter.cs — added SheetIndices / SheetNames to ConversionOptions; Convert(Stream, ...) filters the sheet list before rendering using a HashSet lookup for O(1) membership tests.
• ExcelToPdfConverterTests.cs — 4 new tests covering index filtering, name filtering, precedence, and no-match fallback; added CreateMultiSheetExcel in-memory xlsx helper.
• README.md — documented the new options with examples.

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[Copilot]

Pull request overview

Adds optional per-sheet filtering to ExcelToPdfConverter so callers can convert only selected sheets instead of always rendering the full workbook.

Changes:

• Added SheetIndices and SheetNames to ExcelToPdfConverter.ConversionOptions and filtered the sheet enumeration accordingly.
• Added xUnit tests covering index filtering, name filtering (case-insensitive), precedence, and no-match behavior.
• Documented the new options in the README with usage examples.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/MiniPdf/ExcelToPdfConverter.cs	Adds SheetIndices / SheetNames options and filters sheets before rendering.
tests/MiniPdf.Tests/ExcelToPdfConverterTests.cs	Adds multi-sheet XLSX test helper + new tests for sheet filtering and precedence.
README.md	Documents multi-sheet selection options and examples.

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[Copilot]

There is an extra stray Markdown code fence after the precedence sentence, which starts a new (unclosed) code block and breaks the README formatting. Remove the trailing ``` on this line (or ensure code fences are correctly paired).

Suggested change

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