Support JSON.jl 1.x (and add a Nix dev shell for local testing)

[tanmaykm]

Summary

This branch contains two independent changes plus a patch version bump:

1. JSON.jl 1.x compatibility — Kuber can now be installed and run against JSON.jl 1.x as well as 0.21.
2. Nix dev shell — a reproducible nix develop / nix-shell environment for running the integration test suite against a local kind cluster.
3. Version bump to v0.7.9 for tagging.

1. JSON.jl 1.x compatibility

JSON.jl 1.0 is a StructUtils-based rewrite with one behavioral change that affects Kuber: JSON.parse(str) now returns objects as JSON.Object{String,Any} instead of the concrete Dict{String,Any}. Kuber's conversion helpers (and OpenAPI.jl's from_json) dispatch on the concrete Dict{String,Any}, so parsed values would miss those methods under 1.x.

Fix: route Kuber's five JSON.parse call sites through a small helper that pins the output type:

_parse_json(x) = JSON.parse(x; dicttype=Dict{String,Any})

dicttype is a no-op on 0.21 (already the default) and restores Dict{String,Any} on 1.x — verified to apply at all nesting levels — so a single code path serves both versions. The serialize path (JSON.json(model)) needs no change: JSON.lower is StructUtils.lower on 1.x and OpenAPI.jl (>= 0.2.2) already extends it.

[compat]: JSON = "0.21" -> "0.21, 1". OpenAPI is left at "0.1,0.2" — OpenAPI < 0.2.2 forbids JSON 1, so the resolver cannot mis-pair.

2. Nix dev shell

flake.nix (+ flake.lock) and a shell.nix compat shim provide kubectl + kind and four helper commands — kuber-cluster-up, kuber-proxy, kuber-test, kuber-cluster-down — with an isolated KUBECONFIG so a host cluster (e.g. system k3s) is never read or clobbered. Julia is intentionally not pinned by Nix; it comes from the host (juliaup). Requires a host Docker daemon.

Testing

Ran the full integration suite against a kind cluster (k8s v1.35.0), via the new dev shell:

• JSON 1.6.1 -> 64/64 pass
• JSON 0.21.4 -> 64/64 pass (identical — no regression)

The put!/get/update!/watch round-trips exercise both the parse (dict dispatch) and serialize (JSON.lower) paths.