feat(manifest): add description field for environments

[harshitsaini17]

Description

Adds an optional description field to environments, as requested in #7105, so the purpose of an environment can live in the manifest instead of a comment:

[environments]
test-integration = { features = ["test", "dev-server"], description = "environment for integration tests using a development server" }

pixi info now shows it (only for environments that have one):

        Environment: test-integration
        Description: environment for integration tests using a development server
           Features: test, default

and pixi info --json includes "description" on the environment, omitted when unset.

Changes:

• pixi_manifest: parse description on TomlEnvironment and store it on Environment (list form env = ["feat"] → None). Manifest mutations that rebuild environments (add/remove feature, etc.) keep the existing description.
• pixi_core: Environment::description() accessor.
• pixi_cli: show it in pixi info text and JSON output.
• Schema: description added to Environment in schema/model.py; schema.json and the pyproject schemas regenerated with python model.py.
• Docs: field listed in the [environments] reference with an example.

Fixes #7105

How Has This Been Tested?

• New unit tests in pixi_manifest: description parsed, absent → None, wrong type (description = 42) → expected a string, found integer, and a full-manifest test covering both the table and list forms.
• Updated the 4 existing "unexpected keys" snapshots, which now list 'description' among the accepted keys.
• cargo test -p pixi_manifest -p pixi-build-rust --lib (670 passed), cargo test -p pixi_core --lib (339 passed), cargo test -p pixi_cli --lib (221 passed).
• cargo clippy -p pixi_manifest -p pixi_core -p pixi_cli --all-targets --no-deps -- -D warnings and cargo fmt --all -- --check: clean.
• Schema: python model.py + pytest in schema/ (102 passed, 1 skipped).
• Manual end-to-end with a debug build: pixi info / pixi info --json on a manifest with described, undescribed and list-form environments; pixi workspace environment add keeps the other environment's description; description = 42 is rejected by pixi task list.

Note: pixi_manifest must be tested together with a crate that enables minijinja's json feature (e.g. pixi-build-rust, as the workspace test run does); testing it alone makes the existing tojson_stays_json test fail on main too. That is unrelated to this PR.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

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