fix(build): include Cargo config in pub archive

[reez]

Pub.dev omits the hidden native/.cargo directory, but the build hook passes its config.toml path to Cargo, causing published-package builds to fail.

This moves the config to native/cargo-config.toml, updates the build references, and adds CI coverage confirming the file is included in the publish archive.

[reez]

@Johnosezele (and anyone else that wants to give feedback) does moving the Cargo config to a non hidden path and verifying it in the pub.dev package manifest seem like the right fix, or is there a better Dart packaging convention we should use?

[Johnosezele]

I think moving the file is the right approach. Dart excludes hidden folders like .cargo from published packages, and there isn’t another Dart packaging convention for Cargo config files.

We could keep .cargo/config.toml using .pubignore exceptions, but that’s more fragile. Since the build already tells Cargo where the config is with --config, using native/cargo-config.toml seems cleaner.

Only small suggestion: make the CI check verify the exact archived file if possible, rather than matching any file named cargo-config.toml.

[Johnosezele]

Only tradeoff I see is that direct Cargo builds from native/ will no longer discover the config automatically, so those builds must also pass --config.