chore: add fpdf to API requirements for PDF export

[dj0409]

Description

Added the fpdf dependency to requirements.txt to support PDF report generation in the Harmony Python package. This change is required for compatibility with the new export_pdf_report.py functionality recently introduced in the main Harmony repo (harmonydata/harmony).
This does not affect existing API behavior, but ensures that any code relying on the PDF export logic functions correctly without missing dependencies.

Fixes # (issue)

No specific issue number — support for a new feature from the Harmony library.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Requires a documentation revision

Testing

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Tested the dependency installation locally via pip install -r requirements.txt

• Confirmed that importing and using fpdf.FPDF works without errors

• Confirmed that PDF reports can be generated using the Harmony Python library inside the API environment

Test Configuration

• Library version: latest main of harmony
• OS: Windows 10
• Toolchain: Python 3.11

Checklist

• [x ] My PR is for one issue, rather than for multiple unrelated fixes.
• [ x] My code follows the style guidelines of this project. I have applied a Linter (recommended: Pycharm's code formatter) to make my whitespace consistent with the rest of the project.
• [x ] I have performed a self-review of my own code
• [x ] I have commented my code, particularly in hard-to-understand areas
• [ x] I have made corresponding changes to the documentation
• [x ] My changes generate no new warnings
• [x ] I have added tests that prove my fix is effective or that my feature works
• [ x] New and existing unit tests pass locally with my changes
• [ x] Any dependent changes have been merged and published in downstream modules
• [ x] I have checked my code and corrected any misspellings