[native_assets_cli] Validate conditionally required fields

[dcharkes]

Closes: #1826

This PR adds the final step to the generated syntax validation: conditionally required fields:

1. If target OS is x, then require x config in code config.
2. If target OS is windows, then require windows in the c compiler config (more info [native_assets_cli] Introduce CCompilerConfig.windows #1913).
3. If link mode is dynamic library bundled or static library, then require a file in a code asset.

We could consider trying to nest the fields under the condition, but that has other downsides:

RE 1: Then the OS is no longer an enum usable in the code-asset as OS field. (We could consider this if we remove the OS/arch from code asset outputs. We should be able to do this due to the code config always having a single OS and architecture anyway. #2127)
RE 2: That would mean the compiler config would be split over two places. input.config.code.cCompiler and inputconfig.code.windows.cCompiler. Maybe that's better? Maybe not?
RE 3: Treating a group of files in assets would then become input.assets.code.switch( ... ) instead of simply input.assets.code.map((a) => a.file). Maybe that's okay because we don't often use files in such way anyway?

WDYT @mosuem @HosseinYousefi?

(I'd probably do any of those refactorings in follow up PRs.)

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 85.992% (+0.06%) from 85.936%
when pulling fd8940c on syntax-required-conditionally
into 2fc2bab on main.

[HosseinYousefi]

conditionally required fields

Are these available in json schemas?

[dcharkes]

conditionally required fields

Are these available in json schemas?

Yes, that's where we are generating them from.

Search for "if" in the schemas:

        {
          "if": {
            "properties": {
              "target_os": {
                "const": "macos"
              }
            }
          },
          "then": {
            "required": [
              "macos"
            ]
          }
        },

[HosseinYousefi]

It's slightly inefficient to remove the first element from the list (O(N^2)). Why not just loop over the path?