chore: bump golang to 1.27 and format

[gacevicljubisa]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

Bumps the toolchain to Go 1.27 and applies the modernizations the new toolchain and linter flag. No functional changes.

Toolchain / infra

• go.mod: go 1.26 → go 1.27.0; Dockerfiles golang:1.26 → golang:1.27
• Debian base images 12.12-slim → 12.15-slim
• golangci-lint v2.11.3 → v2.13.2
• CI actions: checkout@v5→v7, setup-go@v6→v7, cache@v5→v6

Code modernizations (49 files, mechanical)

• errors.As → errors.AsType[T] (14 sites)
• pointer helper → new(expr), dropping the local addrPtr
• strings.SplitN(s, sep, 2)[0] → strings.Cut
• flattened embedded struct literals using Go 1.27 field-selector keys
• extracted a repeated "application/octet-stream" literal into a const

Worth knowing

encoding/json is now backed by the v2 implementation.

Also free with this bump: /debug/pprof/goroutineleak now works via the existing pprof route, no code change.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

AI Disclosure

• This PR contains code that has been generated by an LLM.
• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.