fix(dispatch): resolve nested source dependencies declared in workspace

[aoright]

Description

When a build backend produces host or build dependencies as binary matchspec requirements (such as r-statforbiology * from an R DESCRIPTION file or a Python pyproject.toml), nested solves previously had no visibility into workspace-level source declarations and attempted to solve these dependencies as binary packages from remote Conda channels. If the package was only available from Git or a local path declared in the workspace, the solve failed with "No candidates were found".

This PR:

1. Propagates workspace_sources through SolvePixiEnvironmentSpec, ResolveSourcePackageSpec, assemble_source_record, and nested_solve.
2. In nested_solve, checks if any required package name matches an entry in workspace_sources, resolving it as a source package while preserving the backend's version constraints.
3. Ensures walk_and_resolve falls back to workspace_sources when traversing nested dependencies.

Fixes #6925

How Has This Been Tested?

• Added integration test test_nested_source_dependency_resolved_from_workspace in crates/pixi_command_dispatcher/tests/integration/main.rs:
  • Verifies that without workspace sources, solving fails when a nested source dependency is missing from channels.
  • Verifies that with workspace sources, the nested dependency is resolved from source, included in host packages, and added to the top-level solve output.
  • Verifies that version constraints are enforced against the workspace source package.
• Ran cargo test -p pixi_command_dispatcher (174 unit tests, 44 integration 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.