fix(manifest): reject channel in package runtime dependencies

[aoright]

Description

In pixi.toml, package authors could previously specify channel in runtime dependency tables ([package.run-dependencies], [package.run-constraints], [package.extra-dependencies], and [package.run-exports]). However, a built package cannot constrain or add channels to consuming environments; channels are resolved at the workspace/environment level, and top-level environment solves ignore channel qualifiers in package records. This gave package authors a false impression that dependency provenance was constrained.

This PR:

1. Rejects channel in [package.run-dependencies], [package.run-constraints], [package.extra-dependencies], and [package.run-exports] with clear spans and helpful error diagnostics.
2. Preserves support for channel in [package.host-dependencies] and [package.build-dependencies] (where build-time environments can respect them).

Fixes #6747

How Has This Been Tested?

• Added automated unit tests and snapshot assertions in crates/pixi_manifest/src/toml/package_target.rs:
  • test_channel_in_run_dependencies_is_rejected
  • test_channel_in_run_constraints_is_rejected
  • test_channel_in_extra_dependencies_is_rejected
  • test_channel_in_host_and_build_dependencies_is_allowed
• Ran cargo test -p pixi_manifest package_target (all 13 tests passed).

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.
    Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.