fix: eliminate race condition in concurrent mode with custom font

[mimol91]

Description
gofpdf mutates the slice during GenerateCutFont via append-padding in generateChecksum, causing write/write races when the same bytes are shared across Fpdf instances.

This PR fixes the issue by copying byte slice

Related Issue

Checklist

check with "x", ONLY IF APPLIED to your change

• All methods associated with structs has func (<first letter of struct> *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Followed the unit test when,should naming pattern.
• All mocks created with m := mocks.NewConstructor(t).
• All mocks using m.EXPECT().MethodName() method to mock methods.
• Updated docs/*
• Updated example_test.go.
• Updated README.md
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR adds font byte slice cloning to the PDF builder to prevent aliasing issues during custom font registration, and introduces a concurrent test that validates the fix by repeatedly generating PDFs while using a registered TTF font under concurrent access.

Changes

Font Byte Cloning and Validation

Layer / File(s)	Summary
Font byte slice cloning in builder internal/providers/gofpdf/builder.go	The slices package is imported and slices.Clone(font.GetBytes()) wraps font bytes before AddUTF8FontFromBytes, replacing direct slice pass-through.
Concurrent font access test maroto_test.go	Test imports expand to support font setup; a new concurrent test loads arial-unicode-ms.ttf, registers it as a custom UTF-8 font, applies it as the default, and repeatedly adds rows while generating the PDF to exercise concurrent font access.

🎯 2 (Simple) | ⏱️ ~8 minutes

🐰 A slice that was shared, now safely cloned with care,
Concurrent fonts dance without a despair,
TTF bytes flow through the builder so bright,
No aliasing shadows in the PDF light! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	The PR description explains the race condition, the fix, and includes a related issue. However, it does not fully follow the template structure and is incomplete in some sections.	Clarify the technical details of the race condition, verify the related issue link, and ensure the description provides complete context for reviewers to understand the fix's necessity.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically identifies the main change: fixing a race condition in concurrent mode with custom fonts, which aligns perfectly with the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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