fix(profiles): allow patches to add into empty maps and lists

[bhargavikvmpl-2001]

Fixes: #5766
Fixes: #8637

Description

applyProfile re-marshals the parsed SkaffoldConfig to YAML before applying profile patches. Fields such as docker.buildArgs and helm.releases are omitempty, so an empty collection the user declared (buildArgs: {}, releases: []) is dropped from that document. An add into it then fails with invalid path, because the parent no longer exists. The yaml-patch library itself handles add correctly; the parent is just missing by the time it runs.

This PR adds marshalForPatching, which encodes the config to a yaml.Node and then walks the struct alongside the node tree to restore maps and slices that are empty but non-nil. A YAML {}/[] decodes to a non-nil empty value, while an unset field stays nil, so only collections the user actually wrote come back. Types with a custom MarshalYAML are left alone.

A note on the approach: in my comment on #5766 I suggested patching the original YAML node. That doesn't work, because profile fields are overlaid onto the struct before patches run, and older configs are upgraded first. Keeping user-declared empty collections gets the same result without those problems.

User facing changes

Before: with buildArgs: {} (or releases: []), op: add to /build/artifacts/0/docker/buildArgs/profile (or /deploy/helm/releases/-) fails with invalid path.

After: the patch applies, including when several profiles each add into the same empty collection.

A bare buildArgs: (YAML null) still has no parent to add into, consistent with RFC 6902; buildArgs: {} is the way to declare an empty map.

Testing

• TestApplyPatchIntoEmptyCollections covers both issues, including two profiles adding into the same empty buildArgs. It fails on main with the error from Patches operation 'add' cannot add new keys as show in the JsonPatch example #5766 and passes with this change.
• TestMarshalForPatching checks that nil collections are still omitted, an empty map is kept, and an empty slice inside an otherwise empty struct is kept.
• For all 86 example configs under integration/examples and integration/testdata that parse at the latest schema, marshalForPatching output is byte-identical to yaml.Marshal.
• go test ./pkg/skaffold/schema/... ./pkg/skaffold/parser/..., go vet and gofmt pass.

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[bhargavikvmpl-2001]

Hi maintainers, could someone approve the workflow runs for this PR? As a first-time contributor, the unit, integration and lint workflows are waiting for approval. Locally, the unit tests (./pkg/... ./cmd/... ./hack/...) and make linters pass. Thanks!