docs: add Go code style guide for contributors

[zhoward-1]

Summary

• Adds docs/contributing/dev/go/code-style.md

Content:

• Package naming and structure: naming rules, directory layout (cmd/ vs components/), when to use each
• Interface design: define at use site (not implementation), small single-responsibility interfaces, idempotency documentation, registry pattern for extensible sets — all drawn from actual Backend/Registry/JobQueue code
• Logging conventions: zap vs controller-runtime logr, which to use where, consistent field names, log level guidance (no Warn)
• TODO comment format: TODO(#issue): description — explains the CI enforcement and how to comply
• Test organization: file naming, package foo vs package foo_test, test function naming pattern, mock generation with mamockgen, table-driven tests

All code examples are derived from existing production code (inferenceserver/backends/, scheduler/).

Why

error-handling.md exists and is thorough. Contributors working on package structure, interface design, and logging conventions have no equivalent reference. This was priority 9 in the proposal doc.

Test plan

• Verify all internal links resolve (error-handling.md, use-go-mocks-in-unit-test.md, manage-go-dependencies.md, testing.md)
• Confirm mamockgen is the actual mock generator used in the repo
• Confirm mock subdirectory naming convention (backendsmocks) matches actual generated dirs

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