fix(context): handle missing HTML renderer

[Boulea7]

Summary

• avoid a nil pointer panic when Context.HTML is used without configuring an HTML renderer
• pass the missing-renderer error through Context.Render, preserving the HTML content type and existing no-body, error, and abort behavior
• cover GET, 204, 304, and HEAD responses

Fixes #3605

Testing

• go test ./... -count=1
• go test -race ./... -count=1
• go test -tags nomsgpack ./... -count=1
• go test -tags go_json ./... -count=1
• go test -ldflags='-checklinkname=0' -tags sonic ./... -count=1
• go vet ./...
• go build ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.0 run --verbose

Local baseline limitations:

• make fmt-check reports an existing Go 1.26.5 alignment difference in unchanged binding/binding_nomsgpack.go; the changed files are formatted cleanly.
• the locally installed golangci-lint 2.12.2 reports five findings in unchanged code; the CI-pinned 2.11.0 run reports zero issues.

Pull Request Checklist

• Open against the master branch.
• All tests pass in available continuous integration systems (pending).
• Tests are added for the changed behavior.
• Documentation is not required for this bug fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3dc1cd6) to head (524a96b).
⚠️ Report is 289 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4805      +/-   ##
==========================================
- Coverage   99.21%   98.33%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3177       -5     
==========================================
- Hits         3157     3124      -33     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.33% <100.00%> (?)
go-1.26	98.33% <100.00%> (?)
macos-latest	98.33% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.33% <100.00%> (-0.89%)	⬇️

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