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

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.93%. Comparing base (3dc1cd6) to head (1cc6214).
⚠️ Report is 232 commits behind head on master.

Files with missing lines	Patch %	Lines
context.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4491      +/-   ##
==========================================
- Coverage   99.21%   98.93%   -0.29%     
==========================================
  Files          42       45       +3     
  Lines        3182     2992     -190     
==========================================
- Hits         3157     2960     -197     
- Misses         17       23       +6     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.92% <75.00%> (?)
-tags go_json	98.85% <75.00%> (?)
-tags nomsgpack	98.91% <75.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.93% <75.00%> (?)
go-1.25	98.93% <75.00%> (?)
macos-latest	98.93% <75.00%> (-0.29%)	⬇️
ubuntu-latest	98.93% <75.00%> (-0.29%)	⬇️

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