feat(ENG-12409): implement support for the Nix format & upstreams

[apoclyps]

Description

Adds Nix as a supported package format, following the same pattern used to add generic support in #256.

• Bumped the cloudsmith-api dependency from >=2.0.29,<3.0 to >=2.0.30,<3.0 (and regenerated uv.lock) to pick up the generated Nix SDK models (NixPackageUploadRequest, NixUpstream, NixUpstreamRequest, NixUpstreamRequestPatch).
• Added "nix" to UPSTREAM_FORMATS in cloudsmith_cli/cli/commands/upstream.py, enabling cloudsmith upstream nix {create,list,update,delete}.
• cloudsmith push nix required no code change — push.py derives its per-format subcommands and options dynamically via introspection of cloudsmith_api.models for *PackageUploadRequest classes, so it picked up the new NixPackageUploadRequest model (and its --narinfo, --version, --republish, --tags options) automatically once the SDK dependency was bumped.
• Added a CHANGELOG.md entry under [Unreleased].

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Other (please describe)

Additional Notes

Example usage:

$ cloudsmith push nix --help
Usage: cloudsmith push nix [OPTIONS] OWNER/REPO PACKAGE_FILE

  Push/upload a new Nix package upstream.
...
  --narinfo TEXT                The narinfo sidecar metadata file (<storeHash>.narinfo).
                                 Optional — the package can be completed later by
                                 publishing the matching narinfo via the native HTTP PUT path.
  --republish / --no-republish  If true, the uploaded package will overwrite any others
                                 with the same attributes (e.g. same version); otherwise,
                                 it will be flagged as a duplicate.
  --tags TEXT                   A comma-separated values list of tags to add to the package.
  --version TEXT                The raw version for this package.

$ cloudsmith upstream nix --help
Usage: cloudsmith upstream nix [OPTIONS] COMMAND [ARGS]...

  Manage nix upstreams for a repository.

Commands:
  create|new  Create a nix upstream for a repository.
  delete|rm   Delete a nix upstream for a repository.
  list|ls     List nix upstreams for a repository.
  update      Update a nix upstream for a repository.

Verified locally with the above --help output, the full unit test suite (699 passed), and pre-commit run --all-files.

[Copilot]

Pull request overview

Adds first-class CLI support for the Nix package format by updating the generated Cloudsmith SDK dependency and enabling Nix in the upstream command group list, aligning with the existing dynamic format-introspection patterns used by push and upstream.

Changes:

• Bump cloudsmith-api dependency to >=2.0.30,<3.0 (and update uv.lock) to pick up Nix SDK models.
• Add "nix" to UPSTREAM_FORMATS, enabling cloudsmith upstream nix {create,list,update,delete}.
• Document the new support in CHANGELOG.md under [Unreleased].

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
uv.lock	Locks cloudsmith-api to 2.0.30 with updated sdist/wheel hashes.
pyproject.toml	Updates the runtime dependency constraint to cloudsmith-api>=2.0.30,<3.0.
cloudsmith_cli/cli/commands/upstream.py	Enables nix as an upstream format via UPSTREAM_FORMATS.
CHANGELOG.md	Adds an [Unreleased] entry announcing Nix support and the SDK bump.

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