render: add PDF renderer and tests

[Aum-Patel1234]

This PR adds a PDF renderer to the render package, allowing Gin to write
raw PDF bytes with the correct Content-Type.

Includes unit tests to verify header and body behavior.

Fixes #4456

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (3dc1cd6) to head (24013a2).
⚠️ Report is 236 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4491      +/-   ##
==========================================
- Coverage   99.21%   99.00%   -0.21%     
==========================================
  Files          42       45       +3     
  Lines        3182     3022     -160     
==========================================
- Hits         3157     2992     -165     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	99.00% <100.00%> (?)
-tags go_json	98.93% <100.00%> (?)
-tags nomsgpack	98.99% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	99.00% <100.00%> (?)
go-1.25	99.00% <100.00%> (?)
macos-latest	99.00% <100.00%> (-0.21%)	⬇️
ubuntu-latest	99.00% <100.00%> (-0.21%)	⬇️

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[Copilot]

Pull request overview

Adds first-class PDF response support by introducing a dedicated PDF renderer and a new Context.PDF helper, ensuring responses use the correct Content-Type.

Changes:

• Add render.PDF renderer that writes raw PDF bytes with Content-Type: application/pdf.
• Add Context.PDF convenience method to render PDFs via Gin contexts.
• Add unit test covering render.PDF header/body behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
render/render_test.go	Adds a unit test verifying render.PDF sets Content-Type and writes bytes correctly.
render/render.go	Registers PDF for compile-time Render interface compliance.
render/pdf.go	Introduces the render.PDF renderer implementation.
context.go	Adds Context.PDF helper to render PDF bytes with the correct content type.

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[Copilot]

Context.PDF introduces a new public response helper but there are no corresponding tests in context_test.go. Other helpers (e.g., XML, Data) have both “normal” and StatusNoContent coverage to ensure body suppression still sets the correct Content-Type. Please add similar tests for PDF (including the 204/no-body case) to prevent regressions in header/body behavior.

[Aum-Patel1234]

I have added the necessary tests in context_test.go