Use header constants across the handler package

[andrew]

goconst tripped on main after #259 landed on top of #280: five composite-literal occurrences each of "Content-Length" and "Content-Type" across container.go, container_manifest.go, container_tags.go, handler.go, and swift.go crossed the min-occurrences: 5 threshold. Neither PR hit it alone; the CI lint job on #259's rebased head failed but auto-merge proceeded because lint is not a required check.

Add headerContentType and headerContentLength beside the existing headerAcceptEncoding and use them throughout the package rather than only at the flagged sites, so the next handler that adds one does not re-trip the check.

[Copilot]

🟢 Approval recommended

The changes are a mechanical, package-local refactor to shared header-name constants with no functional behavior changes observed.

Pull request overview

This PR standardizes HTTP header key usage across the internal/handler package by introducing shared constants for Content-Type and Content-Length, preventing goconst from flagging repeated header strings and reducing future duplication.

Changes:

• Added headerContentType and headerContentLength constants alongside the existing headerAcceptEncoding.
• Replaced Content-Type / Content-Length string literals with the new constants across handler implementations.
• Updated header-copying/header-proxying loops to use the constants for consistent header key handling.

File summaries

File	Description
internal/handler/handler.go	Defines headerContentType / headerContentLength and uses them in shared handler utilities.
internal/handler/container.go	Switches container blob response header setting/copying to constants.
internal/handler/container_manifest.go	Uses constants when reading/writing/copying manifest Content-* headers.
internal/handler/container_tags.go	Uses constants for cached tag list Content-* metadata and header copying.
internal/handler/swift.go	Uses constants for Swift archive/metadata/problem responses and header copying.
internal/handler/rpm.go	Uses headerContentType for RPM download responses.
internal/handler/pypi.go	Uses headerContentType when serving rewritten HTML/JSON responses.
internal/handler/pub.go	Uses headerContentType when serving proxied/rewritten JSON metadata.
internal/handler/nuget.go	Uses headerContentType when serving proxied/rewritten JSON responses.
internal/handler/npm.go	Uses headerContentType when serving proxied/rewritten package metadata.
internal/handler/hex.go	Uses headerContentType for passthrough and rewritten hex package responses.
internal/handler/helm.go	Uses headerContentType when defaulting chart content type.
internal/handler/gradle.go	Uses constants for Gradle build-cache Content-Type and Content-Length responses.
internal/handler/gem.go	Uses headerContentLength in case-insensitive header filtering logic.
internal/handler/debian.go	Uses headerContentType for Debian package download responses.
internal/handler/conda.go	Uses headerContentType for proxied/rewritten JSON responses.
internal/handler/composer.go	Uses headerContentType for Composer JSON responses.
internal/handler/cargo.go	Uses headerContentType for Cargo config/index responses.

Review details

• Files reviewed: 18/18 changed files
• Comments generated: 0
• Review effort level: Lite

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