libstore: Fix dev environments with CA derivations

[domenkozar]

Summary

• port NixOS/nix@801cb16's type-aware development-environment output handling to BuildEnvironment::getDevEnvironment
• preserve content-addressed and deferred output types and populate their paths with fillInOutputPaths()
• add a functional regression for an input-addressed shell derivation while ca-derivations is enabled

Root cause

The fork moved development-environment extraction from src/nix/develop.cc into libstore after the upstream fix had landed, but the extracted implementation retained the older CA feature branch. When ca-derivations was enabled for an otherwise input-addressed derivation, that branch put Nix's deferred hash placeholder in out. Derivation validation then rejected the generated environment because the variable did not match its computed store path.

Impact

Consumers of BuildEnvironment::getDevEnvironment, including devenv's C API integration, can now obtain development environments when ca-derivations is enabled.

Fixes cachix/devenv#2364.
Related devenv change: cachix/devenv#3044.

Validation

• built the fork and ran nix-functional-tests:ca / develop-input-addressed: 1 passed, 0 failed
• Clang formatting check on the changed C++ range
• Meson formatting check on the changed test registration
• ShellCheck and bash -n on the new test
• Git whitespace checks

The repository-wide pre-commit derivation still reports pre-existing formatting drift in unrelated files; the changed files pass their targeted formatters.